Gaze: ft-gaze's loop runs every 4 ms instead of 2

ft-gaze's loop slept 2 ms, so 500 times a second it read the head pose
(GetDeviceToAbsoluteTrackingPose), checked the eye tracker's counter, and drained SteamVR's
events, for samples that come 90 times a second.

It now sleeps 4 ms. A new sample is printed within 4 ms of appearing, 2 on average (was 1),
and the pose history still has a pose within 2 ms of any sample's time, which keeps the
head-pose error under 0.2 degrees for a head turning 100 degrees a second. Sleeping until
the next sample is due would have thinned the pose history to 11 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

gaze/ft-gaze.cpp

@@ -690,7 +690,10 @@ int main(int argc, char **argv) {
             break;
         }
         if (stdinClosed) break;
-        std::this_thread::sleep_for(std::chrono::milliseconds(2));
+        // 250 passes a second: a new sample is printed within 4 ms (2 on average) of appearing,
+        // and the pose history has a pose within 2 ms of any sample's time (a 0.2 degree head
+        // turn at 100 degrees a second). Every 2 ms read the pose and the events twice as often.
+        std::this_thread::sleep_for(std::chrono::milliseconds(4));
     }
     screens.Stop();
     vr::VR_Shutdown();