Render only the window in the Quest's immersive window

- The pacing thread aims each immersive window eye through the window itself (AimEyesThroughWindow):
  it keeps its position but looks square-on at the window's plane through an off-axis frustum just
  around it, so the eye image is the window, at the display's pixel density (about 680x380 per eye
  at render_scale 0.8 instead of 1344x1408), with a two-pixel border the mask leaves transparent.
  The frame's views carry that pose and field of view to the projection layer.
- vulkan_interop.cpp copies an eye smaller than its AHardwareBuffer into the buffer's corner, and
  the Quest layer's imageRect is the rendered part of the swapchain image.
- These eyes are not foveated: their field of view follows the head, which would rebuild the
  density map every frame.
- Quest only (kWindowShapedEyesSupported); the PC backends copy whole eyes and keep masking them.
  debug.wiicompiled.window_eyes 0 renders them whole and masked again for A/B timing.
- Quest 3, paused Retro Rewind race, render_scale 1.0: Immersive window runs the GPU at level 1
  (456 MHz, app GPU 11.0 ms, eyes 8.4 ms) where Immersive needs level 3 (599-640 MHz, 13.4 ms,
  10.3 ms), about 42% fewer GPU cycles. No edge artifacts, image as sharp. Docs: OPENXR.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/include/vr/openxr_backend.h

@@ -136,6 +136,10 @@ struct OpenXRPresentation {
     // the race's 2D-layer screen (AuroraStereoFrame::window), so the projection
     // layer is blended by its alpha over whatever is under it.
     bool immersive_window = false;
+    // The immersive window's eyes were aimed through the window itself, so each
+    // is only the window: its image is render_width x render_height, the
+    // top-left part of the eye's swapchain image the layer shows.
+    bool window_eyes = false;
 
     // Show the room through the headset's cameras around the virtual screen or
     // the immersive window (OpenXRPassthrough). Taken when the presentation is