Add support for multiple shader models.

We support 1.10 (for OpenGL 2.1 cards), 1.30 (for OpenGL 3.2 core contexts),
and 3.00 ES (for GLES3). There's some code duplication, but thankfully
not a whole lot.

With this, we compile in core contexts without any warning from ATI's driver,
and should also in theory be GLES3 compliant (tested on NVidia's desktop driver).