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.
Fixed the game resizing algorithm and made it much more robust.

This is part of #232 and #206.
The windows resizing algorithm was using a function only working in fullscreen mode. Thanks to bit.dangler from libsdl for pointing out
my own mistake.

The game will now use the desktop current resolution when in fullscreen and back up to a standard resolution if the fullscreen was higher than 1024 in width.
Made the video resolution option be disabled in fullscreen mode as the game is using the desktop one.