OBJ_LOADER_DEMO: Use a triangle (3, 0), (0, 0), (0, 3) for fullscreen pass

And use texCoord for the actual pixelcoords
OBJ_LOADER_DEMO: Allow the user to see fullscreen renders of the gbuffers
COMMON: Allow models to set other textures as shader resources

AKA, diffuse, specular color, normal, alpha, etc.