Update gltf2 + draco extension loader.

[fanzhanggoogle]

Hi Don,
I've updated the branch. Mostly merge with dev and some minor fixes. I've been using this branch to test other gltf exporter with draco extension (e.g. gltf-pipeline, collada2gltf). Let me know how you want to proceed. We hope to get it ready so we could merge to main 3JS repo when the draco extension spec is settled. Thanks.

@donmccurdy

[donmccurdy]

Thanks! I went ahead and merged this, but do have a couple concerns:

1. It appears that isVersionSupported(version, callback) is never invoking the callback. Is this a known issue?
2. I'm nervous about the way we're using a callback and trusting that it will actually be invoked synchronously. I will refactor loadMeshes so that things would not fail if the callback were really asynchronous.

[donmccurdy]

I cleaned up (2) above, so that the geometries can be loaded asynchronously.

See: https://github.com/mrdoob/three.js/compare/dev...donmccurdy:feat-gltf-draco-extension?diff=split

[fanzhanggoogle]

Sorry for the late reply. Thanks for mention that. Since we are in the middle of a new bitstream release, I'll fix that once we settled on the release.

[donmccurdy]

Great! Do you expect that would be days, weeks, or...? Should I try to merge this to three.js in the meantime, or wait until the bitstream update?

[fanzhanggoogle]

@donmccurdy I have a fix for the isVersionSupported, but currently waiting for the Webassembly pull request to be merged PR. Meanwhile, we plan to release the 1.0 in the coming weeks. Most of the work is ready, just need to refine some API.