Move Type::Skin out of the material value range

Type::Skin was declared in the materials block of the graph object
type enum, which gave it the BaseType::Material bit even though
QSSGRenderSkin is a texture-data resource and not a material. As a
result isMaterial() returned true for skins and getBaseType()
reported Material | Resource, so for example the runtime loader's
queryAll(RuntimeLoader.Materials) would return Skin objects.

Move the value into the resources block after SkyMaterial so the type
carries only the Resource bit. To keep things compatible we reserve the
old value and keep the test in isMaterial() ensuring we keep things
working if anyone is using the enum outside Qt.

Task-number: QTBUG-149579
Change-Id: Ie5e1d9009bfeeec7fc558c0e81d9b6cdacc8162d
Reviewed-by: Andy Nichols <andy.nichols@qt.io>