necessary due to the more accurate mouse move events that are useful for
sculpting and painting (at least on Linux/X11, not sure about other platforms).
If the update function takes a while to run, this in turn causes more mouse
move events to be accumulated, making things even slower, .. going into a spiral
of slower and slower redraws.
As a solution I've added a INBETWEEN_MOUSEMOVE event next to MOUSEMOVE. A
MOUSEMOVE event is automatically changed to INBETWEEN_MOUSEMOVE when a
MOUSEMOVE event is added after it. This new event type is only handled by
painting/sculpting operators, everything else can happily ignore it.
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