sdl-video: Move sprite_t conversion to gfx.c

Sprite caching is moved to gfx.c as well.
sdl-video: Move all palette use to gfx.c
sdl-video: Move sprite masking to gfx.c
gfx: Merge sprite caches into single hash table
sdl-video: Merge all sprite drawing functions into one
Merge branch 'gfx-sprite-move'