paint `scroll` using the offscreen decode worker

[totaam]

It is currently disabled since it causes visual corruption: #217 (comment)

[totaam]

scroll draw packets always come first so we only have to ensure that all the previous draw packets have been decoded by the ImageDecoder before we can process a scroll draw.
(I think that we can ignore video because that's usually not used with scroll - or at least, let's deal with that later)

Every draw packet has a sequence number in it, so we can just keep track of the sequence numbers we have submitted for decoding and pop them out when we get the result (or better still: after the paintworker has updated the canvas)
What I don't know how to do is how to asynchronously wait for this sequence number list to become empty.

[totaam]

Perhaps this bug fix helps: Xpra-org/xpra@ff36bc1
Having the wrong flush value would have stalled the updates.

[totaam]

Perhaps the problem does not come from the html5 client: Xpra-org/xpra#4201

[totaam]

Now pretty sure that the html5 code was fine and the bug was in the xpra server: Xpra-org/xpra#4596
Perhaps the html5 client just made it easier to trigger.