Also add map_Kd to the OBJ/MTL material kwargs

[ChuangTseu]

While all the other key/values for the MTL material are provided directly on top of Trimesh's interpretation for SimpleMaterial, then accessible through material.kwargs which is useful for custom handling of the materials, map_Kd isn't. I've had a need for getting the map_Kd file path directly instead of the already loaded PIL.Image.

[mikedh]

Thanks for the PR! I guess this would be accessed through: mesh.visual.material.kwargs['file_name']? That seems like it could be a little ambiguous, what if just added it on as a runtime patch value which isn't the greatest but is probably a little clearer:

img = Image.open(util.wrap_as_stream(file_data))
# add the file name on as a runtime attribute
img.file_name = file_name
material["image"] = img

I was also thinking about making a class LazyImage(PIL.Image) which didn't actually load the image until it was accessed, maybe at some point in the future.