This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
Keep track of MeshInstance and GeometryInstance override materials in the GLTFMesh object.
Ensure all arrays are non-empty to conform with "minItems":1 in glTF spec.
Hugo Locurcio
Rémi Verschelde
Lyuma
K. S. Ernest (iFire) Lee