The new platform agnositic class bgfx::VR manages the
functionality that is shared across the various VR platforms.
The individual platform renderers no longer need
to interface with the internal VR tpyes (OVRRenderI) directly
This greatly simplifies the OVR object's surface area which
is now provided by the VRImplI interface. bgfx::VR now manages
core lifecycle issues of the headset.
The notable renderer API changes are the separation of sensor
sampling and rendering. We need these separate so we can control
the timing (later commit) of camera sampling with finer granularity
than at the start of the video frame.
