Added proper OBJ bump map support

OBJ bump maps are textures for simulating bumps and wrinkles on models. Blender used to author bump maps as normal maps for some reason, so I made it automatically detect if it’s a normal map disguised as a bump map and handle it well, since there are a lot of old Blender OBJ models in the wild. But it also handles bump maps as bump maps normally, so it’s basically the best of both worlds.