In the old model, there was a single rendering thread that moved between
the active sink_screen tasks by priority. If a thumbnail and a main
image window both needed updating, the thumbnail render (which can take
a long time since it is effectively single-threaded) could block
all high-priority updates.

This PR adds private threadpools for low priority vips_sink_screen()
rendering tasks. Now all thumbnails will update in the background as
single-threaded tasks, and will not block foreground updates.