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/include/vr/adaptive_resolution.h

@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+// Adaptive resolution for the immersive eyes ([vr] adaptive_resolution), adapted from
+// heurazy/Wiicompiled_VR-PLUS. Once a second the pacing thread reports how many new eye frames
+// reached the headset and how many it should have had. A second well short of the target lowers
+// the eyes' scale by a step; three seconds in a row at the target raise it again. The steps are
+// slow so a single spike never resizes the eyes, which rebuilds their foveation maps.
+//
+// The eyes are rendered at the scaled size into a corner of the full swapchain image, and the
+// projection layer shows that corner, so the compositor does the upscaling and the swapchains are
+// never recreated. Nothing here depends on OpenXR (tests/vr_adaptive_resolution_tests.cpp).
+
+#include <algorithm>
+
+namespace mkw::vr {
+
+class AdaptiveResolution {
+public:
+    static constexpr float kMinimumScale = 0.7f;
+    static constexpr float kStep = 0.1f;
+    // Below this share of the target a second counts as short, at or above the other as on time.
+    static constexpr float kShortShare = 0.85f;
+    static constexpr float kOnTimeShare = 0.97f;
+    static constexpr unsigned kOnTimeSecondsToRaise = 3;
+
+    float Scale() const noexcept { return scale_; }
+
+    // One second's measurement: `fps` new eye frames against `target_fps`. Off resets to full size.
+    float Observe(float fps, float target_fps, bool enabled) noexcept {
+        if (!enabled) {
+            scale_ = 1.0f;
+            on_time_seconds_ = 0;
+            return scale_;
+        }
+        if (!(target_fps > 0.0f) || !(fps > 0.0f)) {
+            return scale_;
+        }
+        if (fps < target_fps * kShortShare) {
+            scale_ = std::max(kMinimumScale, scale_ - kStep);
+            on_time_seconds_ = 0;
+        } else if (fps >= target_fps * kOnTimeShare) {
+            if (++on_time_seconds_ >= kOnTimeSecondsToRaise) {
+                scale_ = std::min(1.0f, scale_ + kStep);
+                on_time_seconds_ = 0;
+            }
+        } else {
+            on_time_seconds_ = 0;
+        }
+        // Whole steps only, so float drift never leaves the eyes a pixel off full size.
+        scale_ = static_cast<float>(static_cast<int>(scale_ / kStep + 0.5f)) * kStep;
+        return scale_;
+    }
+
+    // The scaled size of an eye `full` pixels across, never below 2.
+    static unsigned Scaled(unsigned full, float scale) noexcept {
+        if (scale >= 1.0f) {
+            return full;
+        }
+        return std::max(2u, static_cast<unsigned>(static_cast<float>(full) * scale));
+    }
+
+private:
+    float scale_ = 1.0f;
+    unsigned on_time_seconds_ = 0;
+};
+
+} // namespace mkw::vr