SDL2 support

This adds support for SDL2.
This only replaces the SDL part of Player, the rest is still with pixman. Blitting to screen and resizing to native resolution(!) is hardware accelerated. Instead of switching to 640x480 in fullscreen SDL2 uses a fake 320x240 surface and resizes this to display resolution (1920x1080 for me). So fullscreen looks much nicer now :).

SDL version is detected via SDL_MAJOR_VERSION.

Only tested with SDL 2.0.0, have to update my libs to 2.0.1 first and retest.

Before merging patches for automake or cmake are welcome to detect SDL2.