fix cast from void* to Gluint

[ChristophHaag]

gcc complained
OsvrRenderingPlugin.cpp:788:44: error: cast from 'void*' to 'GLuint {aka unsigned int}' loses precision [-fpermissive]

[DuFF14]

Thanks. OpenGL path is not working, so this code is going to change soon anyway. Unity casts it this way in their example plugin:
GLuint glTex = (GLuint)(size_t)s_leftEyeTexturePtr;