Stunner - Improve performance for image loading and decoding

There are some quick wins to improve the performance when handling images on the broswer, whch can be easily applied as quick wins.

• Move to asynchronous image loading/decoding, "for free", by moving to the new ImageBitmap API
  *^NOTE: A simple way to start with is about finding the Image#load() and onLoad() methods calls, which are relying on synchronous image decodes, and move to the new API method based on Promises
• Decode on a separate worker
• Consider explicitely closing the ImageBitmap on the code, to allow most effective GC and memory de-allocation by the browser