Moving to SDL2

The main part is about the graphic stack and the remainder is about events: the mousewheel and the textinput which is now distinct from pressedkey and encoded in utf8.

The only regression I've noticed is about the anti-aliasing change. For now, I have to destroy the window, change the gl attributes and then recreate the window. This is quiet disturbing when we change the anti-aliasing level in nwn with the slider since the effect is immediate.

I recommend to read the http://wiki.libsdl.org/MigrationGuide to be aware of the new features SDL2 brings. Some of them could be nice, like fullscreen from the resolution of the desktop.

Furthermore, the textinput mechanism could be more widely use for the console since SDL2 is fully working in utf-8. I'll try to have look and improve that part.