Switch from static gl generator to struct

The struct gl generator is cleaner, and using it will probably fix some possible crashes when using multiple GL contexts on Windows.

Directly invoking `generate_gl_bindings` also allows you to directly specify which GL extensions you may wish to use in the future.

I had to remove some commas because of https://github.com/rust-lang/rust/issues/14240

Note: I could only try the `gl-init` example (which doesn't draw anything on the screen), but I don't see any reason why the examples would break.