Keep vertex attributes when copying `TextureVisuals`.

[eliphatfs]

When I load a GLTF files with multiple objects with vertex colors, the color attribute will be dropped at Scene.dump() due to its calling TriMesh.copy which calls visual.copy that does not copy the attributes.

I propose to deep-copy the vertex attributes like the mesh geometry data in TextureVisuals.copy.

[mikedh]

This sounds good to me! Looks like the tests are failing, happy to merge once those pass.