Fix #4150: Windows doesn't start OpenGL unless a start needs it

wgpu's OpenGL backend on Windows keeps a hidden window on a helper
thread that pumps no messages (gfx-rs/wgpu#10504). Windows hands an
input-language change to every UI thread of the process and waits for
each, so switching the keyboard to another layout hung the whole app,
even while it drew with Direct3D 12. OpenGL was started on every
Windows start, as the fallback.

A Windows start now lists Direct3D 12 alone. OpenGL is still listed
where it is the fallback: after a start crashed on an adapter or while
listing them (#2243), after a restart that left an adapter out, and when
Direct3D 12 lists no adapter at all (Windows 7). WGPU_BACKEND still
picks the backends as before.

docs/development.md

@@ -55,7 +55,7 @@ How the web shell (`apps/vectorcraft-web/src/web.rs`) differs from desktop:
 The canvas is rasterized on the CPU (`vectorcraft-render`, vello_cpu); the GPU (wgpu, through eframe) only composites the canvas texture and draws the UI, so any GPU that can show the window will do. The desktop app picks the window's adapter itself (`apps/vectorcraft/src/gpu.rs`, eframe's `native_adapter_selector`) and logs every adapter it found and the one it uses.
 
 - **Order:** adapters that report they can't present to the window are never tried; then the one `WGPU_ADAPTER_NAME` names; hardware before software (llvmpipe, WARP); the native backends (Vulkan, Metal, DX12) before OpenGL; with Automatic, the GPU the Wayland compositor draws with, then the GPU that drives a display, and among those the one that drives the primary display; then the power preference; then a GPU's DX12 adapter before its Vulkan one (Intel's Vulkan driver made the whole window flicker black, #545), keeping the system's order among equals.
-- **Windows backends:** Direct3D 12, falling back to OpenGL. Vulkan isn't started at all unless `WGPU_BACKEND` asks for it (`WGPU_BACKEND=vulkan`): creating a Vulkan instance loads every installed Vulkan driver into the process, and a faulty one (Intel's `igvk64.dll`, #806) crashed the app before its window appeared. After a start that crashed while it listed the Direct3D 12 adapters, later starts list OpenGL's alone (see the crash bullet below).
+- **Windows backends:** Direct3D 12. OpenGL is listed beside it only when a start leaves an adapter out (a crash or a failed restart) or Direct3D 12 lists no adapter (Windows 7): wgpu's OpenGL backend keeps a hidden window whose thread pumps no messages, and the app hung on a switch to another input language (#4150, gfx-rs/wgpu#10504). Vulkan isn't started at all unless `WGPU_BACKEND` asks for it (`WGPU_BACKEND=vulkan`): creating a Vulkan instance loads every installed Vulkan driver into the process, and a faulty one (Intel's `igvk64.dll`, #806) crashed the app before its window appeared. After a start that crashed while it listed the Direct3D 12 adapters, later starts list OpenGL's alone (see the crash bullet below).
 - **The GPU that drives the display.** A GPU without a monitor is the wrong one even when it says it can show the window: on a desktop with a Ryzen's integrated GPU (no monitor) and an NVIDIA card driving both monitors, power saving picked the integrated GPU, Windows had to present every frame across adapters, and the integrated GPU's driver reset under that: the graphics device was lost, or the desktop's compositor went down and every monitor stayed black for minutes (pdfcraft#378). So Automatic asks the system which GPU drives which display and puts that GPU first; the power preference only orders the rest, and decides alone where the system can't say.
   - **Windows:** `EnumDisplayDevices` (the display devices attached to the desktop, and the primary one), through the `winsafe` crate; the PCI ids in each device id are matched against wgpu's adapters. The registry doesn't tell a user which adapter drives a display, and WMI reports a resolution for a GPU without a monitor, so neither is used.
   - **Linux:** the connected connectors in `/sys/class/drm` and their cards' PCI ids; a built-in panel (`eDP`, `LVDS`, `DSI`) counts as the primary display.