This repairs the `animation.html` sample. Using a JS array is more
natural for web developers and more consistent with the latest
JavaScript bindings in gltfio.
After the native async functionality landed, there was no way for web
clients to be notified that the decoding has finished. This PR changes
the existing `onDone` callback so that it gets called after all textures
have been decoded. (Previously it was called after downloading rather
than decoding.)
This has the side effect of simplifying the API because clients no
longer need to call a finalize function.
Philip Rideout