decode images in worker

[totaam]

See createImageBitmap(): It allows you to decode an image in the background, and get access to a new ImageBitmap primitive, which you can draw into a canvas in the same way you would an element, another canvas, or a video.

[totaam]

Same as #16, needs help.

[totaam]

Bonus of using the worker is that we can move the complicated logic that preserves the paint order to the decode worker.

The overhead of using a worker for small rgb+lz4 packets is probably quite high, but since those tend to be small then that's not a real problem. The gains when decoding h264 or large images are quite significant.

[totaam]

Some reports of latency issues with the worker: Xpra-org/xpra#3229 (comment)

[totaam]

99f9ff2 detects broken implementations of createImageBitmap like Firefox and prints:

we can't decode using a worker: TypeError: WorkerGlobalScope.createImageBitmap: 2 is not a valid argument count for any overload.

[totaam]

Looks like image decoding is single threaded in chrome!?
The decode worker checks timeout when running on a slow connection because it seems to be busy loading the desktop background and doesn't start decoding the test images until it is too late.