Change default compression with 'optimize' CLI to Meshopt

[donmccurdy]

I've come to the conclusion that Meshopt might be the better default for the 'optimize' command. It's much faster to decode, requires a smaller decoder, and avoids some tricky problems like:

• Draco compression increases vertex count #1343
• Draco compression may produce an UNSIGNED_SHORT index buffer with vertex count > 65535 #1370
• Can't compress a model with draco #1342
• Draco compression issue #1215
• Missing 'normalized' flag in Draco encoder calls #1098