Sprites are now rendered directly, rather than being blitted to a master
surface which is then copied to a texture. Every sprite now carries an
SDL_Texture in addition to an SDL_Surface (we need both, since
SDL_Texture doesn't give read-access to the pixel data).
All of the image-manipulation code is now in sdl_draw.c, and the
DrawGraph_* functions are now wrappers. Some games (Daibanchou at least)
use an API other than SACT2/DrawGraph, so these operations should be
generic to avoid duplication.