We were already using jobs for decoding PNG and JPEG files, but we were
doing a join. This add three methods to ResourceLoader that allow
clients to amortize the decoding process across multiple frames, even on
single-threaded platforms like WebGL.
This PR adds async loading to the following demos:
- samples/gltf_viewer (now shows a progress bar in the UI)
- android/sample-gltf-viewer
- web/samples/helmet.html
Fixes#1876.