OpenGL OSX and Linux platform specific context sharing

[DuFF14]

OpenGL support is working on Windows in extended mode. For OSX and Linux support, the platform-specific context sharing code needs to be ported. Specifically, this call to wglShareLists is only available on Windows:
https://github.com/OSVR/OSVR-Unity-Rendering/blob/oglSupport/OsvrRenderingPlugin.cpp#L283

This splits to another branch for OSX support, and I just pushed my most recent commits which haven't been tested.
https://github.com/OSVR/OSVR-Unity-Rendering/compare/oglSupportOSX