Gltf pbrMetallicRoughness not loaded correctly

[slimbuck]

If the gltf material does not specify pbrMetalicRoughness at this point in the gltf parser, then the default values should be used instead (as specified by the gltf spec).

The PlayCanvas material should have metalness enabled in this case and roughness set to 1 (i.e. inverseGloss enabled and gloss set to 1 on the material).

This hasn't been noticed before, because the default PC material happens to look roughly correct.