adjust rendering from JSON

[rpavlik]

Apparently does this somewhat already - so will likely need to be adjusted when the schema changes.

Apparently relevant files:

• https://github.com/OSVR/OSVR-Unreal/blob/master/OSVR/Source/OSVR/Private/OSVRHMDDescription.h
• https://github.com/OSVR/OSVR-Unreal/blob/master/OSVR/Source/OSVR/Private/OSVRHMDDescription.cpp
• https://github.com/OSVR/OSVR-Unreal/blob/master/OSVR/Source/OSVR/Private/OSVRRender.cpp

Note that it looks like it might be hooking in to some existing HMD-specific code in Unreal, which we may or may not want.

[DuFF14]

This has been partly addressed here: https://github.com/OSVR/OSVR-Unreal/pull/13 Still need to parse distortion (and implement), which has its own issue: https://github.com/OSVR/OSVR-Unreal/issues/14