Split plugin into multiple files for D3D and OpenGL/Android rendering paths
Double buffered rendering for Windows and Android. Differs from master branch in that it uses the RenderManager CAPI (instead of C++API) and an extra set of render buffers. Which set of buffers is used for rendering alternates each frame. This should give better performance when ATW is enabled.
D3D and Android-specific rendering sections have been removed from OsvrRenderingPlugin.cpp and split into classes which inherit from an abstract OsvrUnityRenderer.cs class. There is still some platform dependent code in OsvrRenderingPlugin.cpp, but this PR moves a lot of the platform-dependent code to OsvrD3DRenderer.cpp and OSVRAndroidRenderer.cpp
Needs a bit more testing before merging. Requires: OSVR/OSVR-Unity#245 Replaces: https://github.com/OSVR/OSVR-Unity-Rendering/pull/37