Pen tablets: a pen's eraser end selects the Eraser on Linux under X11 and XWayland

On Linux under X11, and under XWayland, the X server lists a pen's
eraser end as a pointer device of its own, and winit 0.30 hands it to
the window as a mouse. Turning the pen over and dragging the eraser
end drew with the active tool, for example a Paintbrush stroke.

The tablet reader now marks a pen device whose name or XInput type
contains "eraser", and Services::stylus carries the pen's end with
its pressure. Once a frame while no press is held and the pointer is
in the window, the canvas selects the Eraser when the eraser end
moves, and it selects the tool used before when the tip moves again.
The return goes through Session::switch_tool, so a perspective grid
hidden meanwhile stays hidden. A tool chosen while the eraser end
stands in stays when the tip comes back. The feed starts afresh when
the pen turns over, so a press of one end does not take the pressure
the other end last touched with. Native Wayland is unchanged.

The switch follows PhotoCraft's sync_eraser_tool
(storytold/photocraft#255, and storytold/photocraft#5043 for the
cleared tool).

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

docs/development.md

@@ -115,7 +115,7 @@ The window runs natively on Wayland (eframe's `wayland` feature) and on X11. On
 - **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. 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.
+- **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. A pen device whose name or XInput type contains `eraser` (in any case) is the pen's eraser end: while the pointer is over the window, the canvas selects the Eraser tool when the eraser end moves, and it selects the tool used before when the tip moves again (`crates/ui-egui/src/stylus.rs`). 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.
 
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