This feature can improve load time when textures are downloaded from the
web on non-filesystem platforms like Android.
More specifically, this allows downloaded texture assets to arrive after
the user calls asyncBeginLoad(), which means that decoding and
downloading can occur concurrently.
Prior to this PR, we already used JobSystem for decoding, but we did not
kick off any jobs until after all assets were downloaded.
Still TBD: add this feature for external vertex data.
Partial fix for #5909.
430f060db2