js: Use synchronous Base64 decoder with atob
js: Use synchronous Base64 decoder with atob

This change switches to synchronous Base64 decoder, using atob and
manual copy step. atob is safe to use because all bytes are valid
Unicode codepoints.

This makes startup a bit faster (~10-20ms out of 100ms latency) and
fixes the odd fetch() performance display in Chrome's network view.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer