Add Raspberry Pi support

[sasq64]

Raspberry PI can do GLES2 / EGL from the console directly.

[mdparker]

Right now, I'm not even maintaining this package. I'm going to revisit it in the future and overhaul it in a way similar to what I did with DerelictGL3. Even so, I know nothing about Raspberry PI, do not own one, and have no way to test anything on it. So it's not something I'm interested in. I'll take any pull requests that add support for it, but I won't be implementing it myself.