video-rice: Use SDL_GLES2 headers and move some defines to OGLPlatform.
video-rice: Modified names of some build vars for clarity w/ upstream.

We can always change them again later. Get some input from upstream.
video-rice: Cleanup with no functional changes.
video-rice: Add some no-op macros for upstream convergence.
video-rice: Revert texture color quality to upstream value.

This switch-case will only be entered if we expose ColorQuality or
TextureQuality rice settings to the user, and the user picks 16-bit.
Since we're not exposing these settings, this is safe to revert,
(and it's probably safe to revert regardless).
video-rice: Add gles2 mipmapping support.

Note the use of GL_ES_VERSION_2_0 for conditional compilation.  Macro
is defined in <GLES2/gl2.h>, which is included by SDL.

The synchronized upstream version is now
https://bitbucket.org/littleguy77/mupen64plus-video-rice/commits/5e67e0998f04e7dd297cafbc48c1d17f5fd9ad43