Initial rewrite of opengl renderer.

Screens are now textured onto quads. OS X joins the party.
When rendering for pica is done using gl, this can be updated to expose a FBO, and then that FBO's texture can be applied to the quads.
For now, it's just because the quad sizes are hard-coded this way. I'll see about adding a uniform for aspect ratio and for translating the screens to their places in an additional PR.