sdl/pixels: make SDL_PIXELFORMAT aliases use the C values directly
sdl/pixels: make SDL_PIXELFORMAT aliases use the C values directly

SDL_PIXELFORMAT aliases were not calculated during compile-time. This
change makes the aliases refer to the C values directly. In SDL2 < 2.0.5,
we define the missing enums ourselves.

Signed-off-by: Lilis Iskandar <f9acb790f2c75b127492316321ece0121b8eed9b@veand.co>

veandco/go-sdl2,veandco/go-sdl2