This is a gltfio API change (!)
Jave clients now need to call destroy on MaterialProvider.
Previously, the Java AssetLoader took over ownership of a native
material provider upon construction, but this was neither documented
nor consistent with the C++ layer. (This is historical; in the past we
did not expose MaterialProvider to Java.)
One motivation for this (aside from API consistency) is that users may
wish to preserve the material cache from one run to another.
Fixes#5132.
fb627d8b66