New sprite features

Renderer switching and patch refactor.
Adds object rendering flags. (See the full list in r_defs.h.)
Adds spritexscale, spriteyscale, spritexoffset and spriteyoffset for objects.
Implements object blend modes. (mobj_t->blendmode, uses AST_ enum. See r_data.h)
Implements floor sprites. (RF_SLOPESPLAT, RF_OBJECTSLOPESPLAT, RF_NOSPLATBILLBOARD, RF_NOSPLATROLLANGLE)
Implements sloped floor sprites. (Manage with P_CreateFloorSpriteSlope and P_RemoveFloorSpriteSlope in Lua.)