Merge initial OpenXR eye gaze tracking support.

[kleinerm]

• Cyclops eye single gaze reporting via XR_EXT_eye_gaze_interaction on all operating systems and capable HMD's.
• Binocular eye gaze reporting, pupil diameter estimates, eye openess estimates on MS-Windows 10+ with capable HTC HMD's.
• help OpenXR for basic setup instructions and references to demos and tests.
• help PsychVRHMD for info about how to use eyetracking, capabilities and limitations.

170 work hours spent, but only less than 120 hours funded. Another win for PTB users, another business fail!