gles2rice: Renamed OGLPlatform.h => osal_opengl.h (upstream convention)

OSAL = Operating System Abstraction Layer.  If we need to make OS-
specific functions, the naming convention should be

    osal_foo_bar(...);

The OS-specific implementation of these functions should be located in

    osal_opengl_gl.h
    osal_opengl_gles2.h
    osal_opengl_gles1.h
    etc.