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Fix T87082: Smooth thickness not working

When using the smooth operator, the thickness would not be
smoothed on the first iteration. This was becasue the inner loop
was set to run `r * 20` times but `r` starts at 0.

The fix makes sure that "Smooth Thickness" works on the first
iteration by using a fixed value of `20` as the loop limit. This
makes sure that for every iteration, we run the smoothing of the
thickness 20 more times.

Reviewed By: antoniov

Maniphest Tasks: T87082

Differential Revision: https://developer.blender.org/D10867