Add adaptive resolution for the immersive eyes

[vr] adaptive_resolution (off by default, in the VR tab) lowers the
race's eye resolution by 10% steps, down to 70%, while new eye frames
fall below 85% of the game's 60 FPS, and raises it again after three
seconds on time. Adapted from heurazy/Wiicompiled_VR-PLUS.

The eyes render into the top-left corner of the full swapchain image
and the projection layer shows only that corner, so the compositor
upscales them and no swapchain is recreated, unlike render_scale. The
Linux same-device bridge now copies an eye smaller than its target;
the Quest's bridge and layer already did. The PC's D3D12 path copies
whole eyes, so the setting is not offered on Windows.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sabhzu6VgoWcHtpSk4C9KZ

runtime/tests/vr_config_tests.cpp

@@ -84,6 +84,9 @@ int main() {
     Require(Parse("[vr]\nrepeat_frames = false\n").vrRepeatFrames == false);
     Require(!Parse("[vr]\nrepeat_frames = 1\n").vrRepeatFrames.has_value());
     Require(!Parse("[vr]\n").vrRepeatFrames.has_value());
+    // [vr] adaptive_resolution: off unless set.
+    Require(Parse("[vr]\nadaptive_resolution = true\n").vrAdaptiveResolution == true);
+    Require(!Parse("[vr]\n").vrAdaptiveResolution.has_value());
 #if defined(MKW_HEADSET_STEAM_FRAME)
     Require(RuntimeConfigFile::kVrRepeatFramesDefault);
     Require(std::string_view(MKW_VR_REPEAT_FRAMES_DEFAULT_TOML) == "true");