GLES2 support

[iamgreaser]

The Raspberry Pi 3 can handle Iceball at maybe 1-2 SPF with llvmpipe.

Very much a matter of putting #ifdef USE_GLES2 in places.

This does rely on me building a not-too-old version of SDL2 from source to compensate for the utter shit that is vidmanx.

Consider this a low priority addition.

[iamgreaser]

Considering closing this issue as there is a working experimental GL 2.1 driver for the Raspi and I think once they fix the stability issues it'll be the way to go.

ARM clients have a separate issue that needs to be fixed. I might go ahead and just fix them myself though.

[asiekierka]

Android and non-RPi devices, though. If it's not a massive undertaking, it would probably be a good idea.

[iamgreaser]

Honestly I'd rather have GL 3 Core support in there before even considering GLES 2. This, and GLES 2 support, would bring the "required, completely different sets of shaders" count up to 3.

GLES 1 could be doable though. I think all the immediate mode stuff has been removed, I could be wrong, but I think it's all gone.