Reduce number of undo steps created by Spryrogimp previews.
First, by grouping them into a single undo.
The implementation adds a function for clearing the
incremental preview drawing that is being executed as an idle task,
and closing an undo group in case the preview left it open (which
would usually be the case). A second way to reduce undos is
by generating a new preview only upon mouse button release of
scales in the UI, and not while dragging the scale.
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