GLTFs need more validation on import

Files failing validation by the official GLTF validator github.khronos.org/glTF-Validator don't necessarily fail import. Such cases should throw exceptions. A particularly severe example is importing a glb with some vertex coordinates being float infinity. This then causes Unity asserts for bounding box creation to fail in-Editor, or in UWP Unity apps causes a crash when Unity tries to create a physics collider from the mesh. The importer should check for such invalid things. It also seems to miss non-unit vectors when unit vectors are expected (also flagged by the validator).