Port OpenGL 2.1 bindings to latest Rust

I'm aware that this is the wrong approach and that I should re-generate
the bindings. However:
1. I've tried generating the bindings and ran into some issues I don't
   have time to look into now.
2. I'm planning to move away from 2.1 anyway (as should anyone else
   who's using it ;) ), so this won't be a problem for much longer.