Add Alpha (Very Unstable) Opcode Replay Buffer.  Provides 75fps headtracking with 20fps, 30fps, and 60fps games with no judder, timewarp, or speed change!

Many things still to do on this.  This is an unfinished alpha feature.  Code is not clean, GUI is not clean, functionality is not clean.  OpenGL mode is fairly untested.  Use at own risk for now.
Merge remote-tracking branch 'cegli/VR-Hydra' into HEAD