Linux: a tablet pen's pressure reaches the tools under X11 and XWayland

On Linux under X11, and under XWayland, winit 0.30 hands a drawing
tablet's pen to the window as a mouse without its pressure. Every pen
stroke reached the tools at full pressure: a Calligraphic brush with
Size on Pressure drew one width, and the Liquify tools' Use Pressure
Pen applied full intensity.

When the window is on X11, the desktop app now reads XInput2 raw
events on a second X connection (x11rb's RustConnection, without
unsafe). A device with an "Abs Pressure" valuator is a pen, unless
its XInput type is TOUCHPAD or TOUCHSCREEN. Its pressure, scaled from
the valuator's range to 0..1, goes into Services::stylus, and another
pointer moving clears it. The canvas uses that pressure when a frame
has no touch force. A press takes the highest pressure of the pen's
latest touch since the canvas last read the feed, and waits up to
5 ms for the touch of a hovering pen. The moves and the release of a
held press keep the pressure the pen last touched with when its lift
has already arrived. Native Wayland is unchanged.

The valuator mapping and the reader come from PhotoCraft's tablet
crate (storytold/photocraft#255), with the pressure axis only.

Docs: README.md, docs/development.md, docs/hardware-parity.md,
docs/target-app-parity.md.

docs/development.md

@@ -114,7 +114,8 @@ The window runs natively on Wayland (eframe's `wayland` feature) and on X11. On
 - **Clipboard owners that never answer (#510):** reading the clipboard waits on the app that owns it: arboard gives up on an X11 owner after 4 s, and a Wayland read waits for the owner to send its data. So on Linux the check that enables the Paste menu items runs on a background thread with a second clipboard handle (`crates/ui-egui/src/clipboard_probe.rs`, installed as `Services::clipboard_probe`), only while nothing is copied in the app and a document is open, and the window keeps drawing. A paste itself still reads on the UI thread, so pasting from such an owner waits for it once. Windows and macOS keep the check on the UI thread: Windows only asks which formats the clipboard holds, macOS asks the pasteboard server.
 - **Copied files:** files copied in a file manager (`text/uri-list`; `CF_HDROP` on Windows, file URLs on macOS) paste as the first one that is art (SVG, PDF, EPS, EMF/WMF or a bitmap; a `.ai` or `.ait` file from the editing data it holds, else as the PDF or PostScript it holds), before the clipboard's other formats (which include the files' paths as text).
 - **Dropping files on the window does not work under Wayland:** winit 0.30, which eframe 0.36 runs on, has no Wayland drag and drop ([winit#1881](https://github.com/rust-windowing/winit/issues/1881), added in winit 0.31; [egui#1563](https://github.com/emilk/egui/issues/1563)), so no `dropped_files` arrive. Until eframe moves to winit 0.31: copy the files in the file manager and paste them, use File › Place or Open, or open the window through XWayland, where drops work: set **Preferences › Performance › Linux Display Server** to **X11 (XWayland)** and restart (#1151), or start the app with `WAYLAND_DISPLAY= vectorcraft`. The preference (`linuxDisplayServer`, read at startup in `apps/vectorcraft/src/main.rs`, shown on Linux only) has winit open the window on X11 (`EventLoopBuilderExtX11::with_x11`) when the session is Wayland, `DISPLAY` is set and the X11 libraries winit loads are installed (`apps/vectorcraft/src/xwayland.rs` reads `ldconfig -p` and the usual library directories); otherwise the window opens on Wayland and the log says why. Unlike clearing `WAYLAND_DISPLAY`, it leaves the clipboard on the compositor's own.
-- **Pens and pen displays under Wayland (#491, #2028):** winit 0.30 doesn't bind the Wayland tablet protocol (`zwp_tablet_v2`; tablet input arrives in winit 0.31, [winit#4318](https://github.com/rust-windowing/winit/pull/4318), [egui#7731](https://github.com/emilk/egui/pull/7731)). Since Plasma 6.3, KWin no longer turns tablet input into pointer input for such apps ([kwin!6336](https://invent.kde.org/plasma/kwin/-/merge_requests/6336), commit [25c65abd](https://invent.kde.org/plasma/kwin/-/commit/25c65abddd95eb87fb892a8594faccbf509ff62f)), and GNOME's Mutter does not turn it into pointer input either (that commit's message): on KWin the pen moves the compositor's cursor over the window, but the app gets no motion or presses and shows none of its own cursors. Until eframe moves to winit 0.31, either run the app under XWayland, which supports the tablet protocol and hands the pen to X11 apps as a pointer (Preferences › Performance › Linux Display Server › X11 (XWayland); X11 (XWayland) When a Pen Is Attached (`x11WithPen`), which opens the window through XWayland only when `/proc/bus/input/devices` lists a pen as the app starts, a device with the `BTN_TOOL_PEN` key and the `ABS_X` and `ABS_Y` axes, and logs `pen found: opening the window through XWayland`; `WAYLAND_DISPLAY= vectorcraft`, or `env -u WAYLAND_DISPLAY vectorcraft`; for a desktop launcher, put `env WAYLAND_DISPLAY=` before the command in its `Exec=` line), or turn KWin's deprecated emulation back on for the whole session by setting `KWIN_WAYLAND_EMULATE_TABLET=1` in KWin's environment (for example in a file under `~/.config/environment.d/` or in `/etc/environment`) and logging in again. The pen then works as a mouse, without pressure: winit 0.30 reads pen pressure on Windows only (Windows Ink, which reaches the Liquify tools' Use Pressure Pen). Other compositors that don't emulate a pointer for tablets behave the same way.
+- **Pens and pen displays under Wayland (#491, #2028):** winit 0.30 doesn't bind the Wayland tablet protocol (`zwp_tablet_v2`; tablet input arrives in winit 0.31, [winit#4318](https://github.com/rust-windowing/winit/pull/4318), [egui#7731](https://github.com/emilk/egui/pull/7731)). Since Plasma 6.3, KWin no longer turns tablet input into pointer input for such apps ([kwin!6336](https://invent.kde.org/plasma/kwin/-/merge_requests/6336), commit [25c65abd](https://invent.kde.org/plasma/kwin/-/commit/25c65abddd95eb87fb892a8594faccbf509ff62f)), and GNOME's Mutter does not turn it into pointer input either (that commit's message): on KWin the pen moves the compositor's cursor over the window, but the app gets no motion or presses and shows none of its own cursors. Until eframe moves to winit 0.31, either run the app under XWayland, which supports the tablet protocol and hands the pen to X11 apps as a pointer (Preferences › Performance › Linux Display Server › X11 (XWayland); X11 (XWayland) When a Pen Is Attached (`x11WithPen`), which opens the window through XWayland only when `/proc/bus/input/devices` lists a pen as the app starts, a device with the `BTN_TOOL_PEN` key and the `ABS_X` and `ABS_Y` axes, and logs `pen found: opening the window through XWayland`; `WAYLAND_DISPLAY= vectorcraft`, or `env -u WAYLAND_DISPLAY vectorcraft`; for a desktop launcher, put `env WAYLAND_DISPLAY=` before the command in its `Exec=` line), or turn KWin's deprecated emulation back on for the whole session by setting `KWIN_WAYLAND_EMULATE_TABLET=1` in KWin's environment (for example in a file under `~/.config/environment.d/` or in `/etc/environment`) and logging in again. The pen then works as a mouse. Under XWayland the app also reads the pen's pressure (next item); with KWin's emulation on native Wayland the pen has no pressure. Other compositors that don't emulate a pointer for tablets behave the same way.
+- **Pen pressure under X11 and XWayland (#764):** winit 0.30 hands a pen to an X11 window as a mouse, without its pressure. When the window is on X11, XWayland included, the desktop app opens a second X connection (x11rb's `RustConnection`) and reads XInput2 raw events for every device (`apps/vectorcraft/src/tablet.rs`, from PhotoCraft's `crates/tablet`). A device with an `Abs Pressure` valuator is a pen, unless its XInput type is `TOUCHPAD` or `TOUCHSCREEN`. While a pen is the pointer that moved last, its pressure, scaled from the valuator's range to 0..1, reaches the tools that use pressure: the Paintbrush's pressure-sensitive brushes and the Liquify tools' Use Pressure Pen. The X server needs XInput 2.1 or later; otherwise the log has a `tablet pressure:` warning with the reason, and the pen draws at full pressure.
 
 ## Fonts: craft-fonts (optional build input)