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
0eea92c8fc