Initial port to SDL2. DON'T TRY THIS IF YOU DON'T KNOW WHAT YOU'RE DOING!

As suspected, the editor map rendering is dead. Another proof of the urgency
to replace it with proper Qt renderer support.

More annoying, the font are rendered weirdly. I opened a issue about this:
https://bugzilla.libsdl.org/show_bug.cgi?id=2104

- The window resizing is also off. 4144 told me to use glViewPort(). I'll try
  in another commit.
- The gamma correction algorithm is to be redone.
Added a naive desktop fullscreen mode. For now, the screen isn't centered.