I/O readJSON() method cannot decode Data URIs

[donmccurdy]

DracoMeshCompression doesn't handle Data URIs; in general there should probably be an abstraction for extensions that need to access binary data from a buffer before it's parsed into accessors:

Looking into this more, the Draco extension is probably doing the right thing — decoding Data URIs is handled in the I/O class, not the readers. Currently the read('path/to/file.gltf') methods will decode Data URIs, but readJSON does not. That'll be a small breaking change for WebIO unfortunately, since readJSON needs to become asynchronous to support this.

Perhaps it's worth thinking a little more about the sync/async differences between these methods. Currently NodeIO (1) uses sync for everything, and (2), can't load resources over the network. I think I'm OK with leaving (2) to the user but they shouldn't have to manually replace Data URIs. And (1) is not ideal for anything running on a web server.

[donmccurdy]

On further thought, not necessarily a breaking change since we can use BufferUtils.createBufferFromDataURI synchronously. The idea that NodeIO should become async is probably still worth doing at some point though...