Morph targets are now stored in meshes.

Previously, morph targets were stored as a Handle<Image> in a Mesh. Now, morph targets are stored inside the Mesh itself.

As a consequence, Gltf assets no longer provide a GltfAssetLabel::MorphTarget subasset. This subasset can be replaced with the corresponding GltfAssetLabel::Primitive to look up the correct Mesh, followed by Mesh::get_morph_targets.