smarter merging

[freeman-lab]

When we currently merge geometries inside scene here -- e.g. taking all path and center geometries that make up the platform and turning it into one geometry -- we are not discarding points or faces at the boundaries between adjacent geometries. Instead, we are just using the basic mesh-combine module, which literally concatenates all vertices and faces.

Instead, if you have e.g. two rectangles flush side by side, you want to discard all four faces and four of the eight vertices where they intersect. And just do that in general for any adjacent geometries.

The main reason we want this is to prevent the subtle but noticeable "tearing" at the boundaries. I tried some dumb things (like removing duplicate vertices after correcting for rounding) but couldn't quite get it to work.

cc @sethvincent @mathisonian