Long-term: Iron out Android, iOS, OSX portability.

[mauvecow]

Confirm that we can build and properly support all of the target platforms that SDL2 has to offer us.

This means:

• Android and iOS builds have not been tested, no idea if they actually work.
• OSX has not actually been tested by me, or anyone as far as I know. (Time to actually buy some hardware, I think.)
• Any OpenGL tweaks necessary to build.

It's likely that the current, ifdef-y hacks all around the GL code for ES2 support should also be cleaned up into better, separate pathways for ES2 and classic OpenGL.

[mauvecow]

Additionally: I have feedback from a Raspberry Pi user who could not initialize the OpenGL backend, so we need to properly fallback to GLES and, ultimately, a dummy driver if none are available.