lib/sdl2: wrap some SDL 2 system and window manager features

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/sdl2: wrap the SDL 2 image library

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/sdl2: intro the entry module for sdl2

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/sdl2: intro a module uniting for sdl2 and its image library

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/sdl2: intro an example for the sdl2 modules

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/sdl2: add README

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>