to a higher bit depth resulted in 0s. This caused banners to be transparent
on Macs. This also fixes scaling: scale colors instead of truncating them.
This is a little slower; I might need to add an optimized blit for the d3d movie
code path ...
Most of SDL_utils is moved to RageSurfaceUtils, and does not require
SDL; SDL_utils is now only utils that actually need SDL.
(My goal isn't to eliminate SDL; it's to make it a back-end library, that
can be disabled on some platforms, rather than a required library as it
is now.)