OSVR-Unity Android app crashes when multithreaded rendering is enabled

[DuFF14]

Looking into it. Have to make sure the Unity context is current when calling RM functions.

[DuFF14]

multithreaded rendering on android now works on this branch: #245 with https://github.com/OSVR/OSVR-Unity-Rendering/tree/d3d_capi_db_refactor