blit: Add a case for 8-bit blits that sdl12-compat exposed. sdl12-compat can get into a state where a color-keyed surface is marked for blending, but wants to blend with full alpha (which is the same as _not_ blending), so rather than fail to find a blitter in that case, it just selects the colorkey blitter. Reference https://github.com/libsdl-org/sdl12-compat/issues/233