Switch to modern GL /w GLES compat

[fgenesis]

Get rid of the GL matrix stack and all the old GL 1.x cruft.
Might want to make a dedicated renderer subsystem first, move all GL code there, and then transition to at least GL 2.0 with extensions, possibly GLES compatible.

This has sort of been done already (https://bitbucket.org/mattbierner/ios-aquaria), but with apple-specific extensions and not nicely contained in an extra subsystem.