Ensure a 4-dimensional image is loaded

[glambrechts]

Prevents the following crash when loading textures for the first time (on macOS at least) with the command play_craftax.

File "./craftax/craftax/constants.py", line 600, in load_texture
    if jnp_img.shape[2] == 4:
IndexError: tuple index out of range

[glambrechts]

I made sure a similar fix works for craftax_classic too.