InstancedMesh support

[genemerewether]

Hi, would it be possible to add InstancedMesh support with a per-instance .instanceMatrix? My use case is displaying a large number of ellipsoids (many more than geometry.Ellipsoid can handle).

I'm also happy to take a stab at this if there's a preferred way to implement.

[genemerewether]

@rdeits @RussTedrake any thoughts on this?

[siddancha]

Seconded. InstancedMesh would also be super useful to visualize voxel grids (collection of Box geometries) in Meshcat!