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.
Really lazy implementation for now, just draws all sprites every frame.
In the future should implement proper compositing with damage tracking.
All CG types other than QNT are removed. From what I've seen, System 4
games use QNT, PNG, AJP (encrypted JPEG) and DCF (seems to be a format
for partial CG updates). QNT is by far the most common format.