[SDL] Optimal mode from SDL_GetVideoMode

[Ryan C. Gordon]

> Keep in mind that plain X11 is always software rendered, and therefore
> not terribly fast (but fast enough for most uses). To remedy this, you
> must either 1) use OpenGL instead of the 2D API, or 2) wait for SDL
> 1.3, which will provide an OpenGL backend for the 2D API.

Yeah, the X11 target returning 0 for available video memory is not a 
bug; there are no hardware surfaces.

While it's not a fast path, it should _not_ kill your performance, 
generally...I have yet to see a 2D game that couldn't manage with 
software surfaces, even when doing a fair bit of alpha blending. Usually 
the bottleneck is converting data formats during blits, so be sure to 
preconvert them! Otherwise, never call SDL_UpdateRect or SDL_Flip until 
forced to.

Or use OpenGL, of course.

--ryan.