Fix node transformations for the skinning

According to the GLTF2 spec,
"Implementation Note: Client implementations should apply only the transform
of the skeleton root node to the skinned mesh while ignoring the transform
of the skinned mesh node."

Change-Id: Id0225978da899762503d6658343e9dd6af53330c
Reviewed-by: Antti Määttä <antti.maatta@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>