video-rice: Replace some line comments (//) with block comments (/**/).

This makes downstream look much more like upstream when using diff
tools.  When giant chunks of code are commented out with line comments,
it's hard to determine if anything inside that block has changed.
Replacing with block comments makes it much clearer exactly which lines
have changed.
video-rice: Use #ifdef to conditionally include opengl/gles2 libraries.

The GL abstraction occurs in OGLPlatform.h.  Also created some macros
to map function and constant symbols from OpenGL to GLES2, wherever
their syntax differs.