Hand Android's develop frame to the compositor as a texture again

With Skia drawing pre-rotated on wgpu's Vulkan swapchain, Android no
longer needs to draw with Skia over OpenGL, which was the only reason
the develop view read its frame back through memory (TD-1).

So `unstable-wgpu-29` moves back to the common slint dependency. The
android-activity backend then builds `SkiaRenderer::default_wgpu_29`,
and `shared_gpu` loses its Android arm. The one wgpu device is handed to
Slint through `BackendSelector::require_wgpu_29` on both platforms.
`slint::android::init_with_event_listener` runs before `dr_ui::run`, so
the selector reaches the Android adapter before its window exists.
`renderer-femtovg-wgpu` stays desktop-only, since Android has no FemtoVG.

The two `#[cfg(target_os = "android")]` readbacks in `develop::render`
(the frame through `export_pixels` and the focus overlay through
`read_overlay`) are gone. `read_overlay` stays for the tests that check
what the overlay marks.

Built for arm64 and release-signed. Not yet run on the tablet.

core/dr-gpu/src/lib.rs

@@ -6,7 +6,7 @@
 //! module doc said for eight releases that it held no pipeline and no masks.
 //! It holds both now, plus demosaic, detail, segmentation masks, two
 //! histograms and focus peaking. The zero-copy claim is still the one that
-//! matters, and TD-1 records the one platform where it does not hold.
+//! matters, and since TD-1 was paid off it holds on Android too.
 //!
 //! Deliberately free of UI dependencies (ARCH §6.5a). The texture is handed
 //! out as a `wgpu::Texture`; who composites it is not this crate's concern.