Add VR display auto-calibration logic
Add VR display auto-calibration user message calculations
Make overlays accessible from C++ code
Add some property setters to the billboard overlay
Display a static billboard prompting user to hold still to calibrate HMD
Fix avatar rotation erroneously initiating HMD calibration
Delete calibration message if exit VR display while calibrating
Fix header declaration
Make calibration message update its location more often
Update SVG file location
Use const parameters
Use NULL rather than 0 for "no overlay"
Tidy up log messages
Fix building without Oculus library
Merge branch 'master' into 20073
Merge pull request #3579 from ctrlaltdavid/20073

CR for Job #20073 - Automatically calibrate the Oculus Rift after startup