Add some kind of progress bar for PNG download

[fregu856]

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[j6k4m8]

I think this will need to be converted to its own thread — our experiment yesterday shows that even on a beefy machine, this is a showstopper when it runs on the API threads!

My proposal is something like this:

• Add an Export runner
• Add progress the same way we currently handle it for the other runners
• Chunk the downloads so that we can zip and send the first few slices while we process the rest
• Throttle CPU usage so we can run other things in the meantime (i.e., downloading results isn't as high priority for our available cores as segmentation)