Speed up removeUnusedVertices

[lasalvavida]

removeUnusedVertices and by proxy mergeDuplicateVertices are a major bottleneck for speed in the main pipeline right now.

My impression is that removeUnusedVertices is so slow because mergeDuplicateVertices ends up creating lots of holes in the buffer that need to be closed. In order to close those holes, the entire buffer has to be moved over which can be very time consuming for large models.

This could be faster by dividing the buffer up into chunks, closing the gaps in the smaller chunks, then concatenating the used parts of the chunks. It may also be faster to iteratively read through and copy chunks into a new buffer, though probably less memory efficient.

[mramato]

I ran into this today (specifically mergeDuplicateVertices being slow). I have a ~500mb obj model that I'm running through obj2gltf and then gltf-pipeline and if I comment out the call to mergeDuplicateVertices in Pipeline.js, it converts in ~150 seconds. If I run with mergeDuplicateVertices, it never completes (I killed it after 30 minutes). @lilleyse suggested we disable mergeDuplicateVertices by default until this issue can be resolved. I'm going to open a PR to put it behind a flag.