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/src/vr/openxr_vulkan.cpp

@@ -935,10 +935,12 @@ public:
             views[eye].pose.position = frame.xr_frame.views[eye].pose.position;
             views[eye].fov = frame.xr_frame.views[eye].fov;
             views[eye].subImage.swapchain = retained_swapchains_[eye].handle;
+            // The part of the image the eye was rendered into: all of it, except for the immersive
+            // window's eyes, which are only the window (OpenXRPresentation::window_eyes).
             views[eye].subImage.imageRect = {
                 {0, 0},
-                {static_cast<int32_t>(retained_swapchains_[eye].width),
-                 static_cast<int32_t>(retained_swapchains_[eye].height)}};
+                {static_cast<int32_t>(std::min(frame.render_width[eye], retained_swapchains_[eye].width)),
+                 static_cast<int32_t>(std::min(frame.render_height[eye], retained_swapchains_[eye].height))}};
             views[eye].subImage.imageArrayIndex = 0;
         }
         XrCompositionLayerProjection projection{XR_TYPE_COMPOSITION_LAYER_PROJECTION};