[SDL] transparent overlays and rgba->rgba blitting (patch?)

[Sam Lantinga]

> It seems like no one has found the current RGBA->RGBA behavior useful.

The current RGBA->RGBA behavior was designed for SDL_OPENGLBLIT (obsolete)
where you want to define a HUD and blit updates into it without destroying
the alpha channel.

I won't be changing that behavior in SDL 1.2, since too many applications
rely on those semantics, however alpha blending semantics are open for 
discussion for SDL 1.3

See ya!
	-Sam Lantinga, Software Engineer, Blizzard Entertainment