gltfio can now ask the material provider plugin which `Material` would
be used for a given set of requirements. Prior to this change, gltfio
could only create a new `MaterialInstance`.
This method is necessary to support an upcoming gltfio feature.
We still use resgen for convience, but the archive is now passed in
from the client application.
This will allow us to shrink the gltfio Android library (stay tuned).
