WIP EGL support

[kvark]

Attempt to fix #10

I think the first commit has useful stuff that could be merged. The second commit is garbage, and I don't think it's productive for me to continue.

[kvark]

Currently asserts on this line:

"namespace" => assert_eq!(value, "GL"),

EGL spec uses a namespace per enum, so clearly the expectations are different here :/

[Lokathor]

Ah, that's more of a debug assert sort of thing. Feel free to change that if it's giving you trouble. A lot of the checks are just because I was being extra cautious during development as I was building up my own personal understanding of how gl.xml works.