gltfpack: Fix processing for files with clearcoat textures
gltfpack: Fix processing for files with clearcoat textures

Roughness/normal texture wasn't participating in the usesTextureSet
function which meant that we could discard UV channels if they were
*only* used by these maps - which might be uncommon but does happen in
examples from glTF-Sample-Models.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer