Centre foveation on the player's gaze on headsets with eye tracking

With [vr] eye_tracked_foveation (on by default on the Steam Frame, off
elsewhere) the runtime asks for XR_EXT_eye_gaze_interaction. When the
system reports an eye tracker, OpenXRInput binds the gaze pose and
locates it for each packet's display time, in the space the eye views
are located in; vr/eye_gaze.h turns it into tangents of each eye's own
view, which AuroraStereoFrame now carries (appended, after the existing
prefix).

Aurora centres the eye's fragment density map on the gaze snapped to a
cell of two map texels (about 3 degrees). Each eye keeps up to 32 maps,
one per cell looked at, so a glance back reuses its map; a new map is
bound once its upload completes, and until then the eye keeps the map
it had. Without a tracked gaze (a blink, no tracker, the setting off)
foveation centres on the forward direction exactly as before: the
forward maps are byte-identical.

Also logs every extension the OpenXR runtime offers at startup, so the
first Steam Frame session shows what SteamVR's Android runtime has.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

runtime/src/settings_overlay.cpp

@@ -156,6 +156,7 @@ bool g_vrHandTracking = RuntimeConfigFile::VrHandTracking();
 constexpr std::array<const char*, 4> kVrFoveationLabels{"Off", "Low", "Medium", "High"};
 static_assert(kVrFoveationLabels.size() == RuntimeConfigFile::kVrFoveationLevels.size());
 int g_vrFoveation = static_cast<int>(RuntimeConfigFile::VrFoveationLevelIndex(RuntimeConfigFile::VrFoveation()));
+bool g_vrEyeTrackedFoveation = RuntimeConfigFile::VrEyeTrackedFoveation();
 #endif
 bool g_vrFirstPerson = RuntimeConfigFile::VrFirstPerson(false);
 bool g_vrFirstPersonToggleClick = RuntimeConfigFile::VrFirstPersonToggleClick();
@@ -1523,6 +1524,15 @@ void DrawVrSettings() {
                         "lenses blur the picture anyway, to free GPU time. This session started with it "
                         "off, or without a GPU that supports it: a new level applies after a restart.");
     }
+    if (ImGui::Checkbox("Foveation follows the eyes", &g_vrEyeTrackedFoveation)) {
+        RuntimeConfigFile::SetVrEyeTrackedFoveation(g_vrEyeTrackedFoveation);
+    }
+    if (ImGui::IsItemHovered()) {
+        ImGui::SetTooltip(
+            "With a headset that tracks the eyes (the Steam Frame), the sharp centre of the foveated "
+            "race view moves to where you look instead of staying straight ahead. Turning it off "
+            "applies immediately; turning it on needs a restart if the session started without it.");
+    }
 #endif
     {
         const auto rateLabel = [](uint32_t hz) {