Sdl2 screenshot implemented with glReadPixels

Not for merging right now!

I implemented this a while ago, and made the pr for easy reference. This code doesn't quite work, because the flipVert function doesn't flip the texture properly - there's an offest or something, and you get a slant instead. It's probably a small fix, and it works otherwise.

Fixes https://github.com/kivy/kivy/issues/2514.