The vertex data for glTF morph targets are deltas rather than final
values, so we were passing incorrect data into the SurfaceOrientation
helper.
This has been a long-standing bug that affected models with morphing,
including Buster Drone. However in practice we have observed visual
issues only with AnimatedMorphCube.
Tested against AnimatedMorphCube, Buster Drone, Cesium Man, Fox, and
Littlest Tokyo.
Fixes#1609
eb40fa753f