Update meshcat to get better .DAE handling

[rdeits]

Includes meshcat-dev/meshcat#51

cc @tkoolen

[codecov-io]

Codecov Report

Merging #104 into master will increase coverage by 2.04%.
The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage    75.2%   77.25%   +2.04%     
==========================================
  Files          13       14       +1     
  Lines         363      400      +37     
==========================================
+ Hits          273      309      +36     
- Misses         90       91       +1

Impacted Files	Coverage Δ
src/geometry.jl	95.65% <ø> (+2.31%)	⬆️
src/objects.jl	100% <ø> (ø)	⬆️
src/MeshCat.jl	30.43% <ø> (ø)	⬆️
src/lowering.jl	81.69% <100%> (+0.26%)	⬆️
src/mesh_files.jl	95% <95%> (ø)
src/arrow_visualizer.jl	100% <0%> (ø)	⬆️

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[rdeits]

Ok, made a bunch of progress on this today. I ended up having to introduce a new type: MeshFileObject, which is similar to MeshFileGeometry in that you can load it from a mesh file, but different in that it treats that file as representing an entire scene (an "object" in threejs speak), complete with textures, materials, and potentially multiple objects (for .dae files).

The way this works is that you can either create a MeshFileGeometry if you just want to treat the file as if it contains a single blob of geometry (for example, if you want to apply your own material), or you can create a MeshFileObject if you want to let the mesh set its own materials and so on.

For our URDF loading, I think we should always use MeshFileObject for .obj and .dae files. I've included enough logic on the Julia side to take an .obj file, parse it to figure out which .mtl file it uses, and then parse that .mtl file to figure out which image textures it needs, and then wrap up all of those and send them to javascript. I've done the same for .dae files, although at least they don't do the silly .mtl nonsense.

[rdeits]

On the way, I had to do some hacking at the threejs internals (it doesn't actually expose the callbacks we need), and I even learned how to write meshes with image textures by hand. Whee!

[tkoolen]

This all looks great to me!