Simplify Draco and Meshopt compression implementations

[donmccurdy]

Meshopt

Currently glTF Transform's Meshopt implementation has its own accessor writing method, and doesn't support VertexLayout options. I think possibly glTF Transform could just write Meshopt-compatible buffer view layouts by default, and simplify the Meshopt implementation accordingly.

• fix(functions): Group meshopt-compressed accessors by parent #1384

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Draco

The Draco implementation currently has to duplicate accessors in rare situations, leading to complex bugs. It may be worth a change to require pre-processing with draco(), and failing if Draco compression is attempted when that pre-processing has not been applied. Or other simplifications may be possible.

• fix(functions): Avoid failure when Draco generates new accessors #1385
• Can't compress a model with draco #1342
  • Idea: Pre-process to ensure accessors are only reused in draco-compatible ways
• Draco compression may produce an UNSIGNED_SHORT index buffer with vertex count > 65535 #1370
  • Idea: Pre-process to use uint32 indices, let draco do its thing