Switch to using IOSurface instead of CVOpenGLTextureCache on OSX. The latter can't be
used anymore to do YUV => RGB with opengl3 on El Capitan as GL_YCBCR_422_APPLE
has been removed from the opengl3 driver. Also switch to NV12 from UYVY, which
was the only YUV format supported by CVOpenGLTextureCache.
First of a few commits to stop using CVOpenGLTextureCache on OSX and use
IOSurfaces directly instead. CVOpenGLTextureCache hasn't been updated for OpenGL
3 which is why texture sharing is currently disabled on OSX.