Since the `usd_path_to_mat_name` map seems to always be used in tandem
with `mat_name_to_mat` to get the material, change it from a map of
[string, string] to [string, Material] and store the Material directly.
This removes the need to do a secondary lookup through `mat_name_to_mat`
and less memory will be used at runtime due to the smaller Value size.
Pull Request: https://projects.blender.org/blender/blender/pulls/132647
4d5be451a7