Move image resizing into WebWorkers

[workhorsy]

This will have many advantages:

1. Save on memory, as we can destroy the WebWorker when done. This will free all the memory without having to wait for GC on the main thread.
2. Will stop the UI from being jerkey, because it will not have to block it while resizing the images.

[workhorsy]

This will require that we can use the Image object inside WebWorkers.

[workhorsy]

See this list for what is available in WebWorkers: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Functions_and_classes_available_to_workers