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.
Please be more specific here. Which texture? What do you mean with "accounting for source and dest"? TODOs need to make sense to everyone, not just the author.