This adds some significant JNI glue in order to make the Java API for
gltfio materials much more consistent with C++, and create feature
parity between the two languages.
The motivation is to allow Java clients to provide their own
ubershader-based solution or filamat-based solution.
- The `MaterialProvider` Java class is now an interface.
- The `UbershaderLoader` Java class is now a concrete implementation.
- Users can now write their own provider class using Java / Kotlin.
To streamline the amount of JNI work that occurs at run time, the
MaterialProvider Java interface uses ints in a couple spots instead
of enums. This sacrafices some readability for Java clients who wish
to write their own provider, but makes the JNI much simpler.
This replaces #3975.
