Shape layers: change the canvas compressor delay depending on image size
This sort of keeps the best of both the fix for
https://bugs.kde.org/show_bug.cgi?id=399363 and some responsiveness
for smaller images.
The real solution probably would be to render the vector shapes at
the current zoom level, LoD-like (and only for the visible part,
of course), but that's way out of scope for now. We might as well
wish we could write our svg renderer without QPainter and with
proper color management.