Legacy OpenGL 1.x backend

[JayFoxRox]

There should be a GL1.x backend (or something similar). While this might sound stupid and ridiculous, it would make it easier to port to more platforms. We don't benefit anything from GL3.3 core and if people want to enhance graphics, they can easily switch back to GL2+ (or rewrite the whole games renderer altogether).

The idea would be to target something like nanogl. Something which simply serves as a stable API for the graphics backend while preserving all features of the game.

This issue should also serve as a discussion platform where we can list technical requirements the legacy API would have to provide (e.g. subset of GL1.x functions / features we'll need + extensions which help with the rest).