MorphPrimitivesTest.gltf morphs positions but not normals, lighting on morphed mesh looks incorrect

[droettger]

The model MorphPrimitivesTest.gltf provides position, normal, and texcoord0 attributes but only morphs positions. As consequence the morphed mesh uses normals of the flat mesh with all normals pointing to (0,1,0) and the lighting looks incorrect. This would not happen if the model wouldn't provide normal attributes (resulting in face normals calculated on the morphed positions) or proper morph targets for the normal attribute.