Decouple Rift head tracking from avatar's head position

This makes the Rift's view reflect the user's physical movements exactly
while the avatar follows the movements as best it can.
Fix upright head model position calculation for avatar scale changes
Position Rift camera at avatar model's mid-eye location
Fix missing constructor initialization
Merge pull request #3471 from ctrlaltdavid/20055

Code Review for Job #20055