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

aurora-main/include/aurora/aurora.h

@@ -209,6 +209,11 @@ typedef struct {
   // everywhere else, for the host's compositor to show its own background
   // (the room, on a headset with passthrough) around it.
   bool window;
+  // Eye-tracked foveation (aurora_set_stereo_foveation), while gazeValid: where the player looks, in
+  // tangents of each eye's view (x right, y up, as in the projection's frustum). The immersive eyes'
+  // full-density region then centres there instead of on each eye's forward direction.
+  float gaze[AURORA_STEREO_EYE_COUNT][2];
+  bool gazeValid;
 } AuroraStereoFrame;
 
 /**