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

source/blender/editors/gpencil/gpencil_edit.c

@@ -3951,7 +3951,7 @@ static void gpencil_smooth_stroke(bContext *C, wmOperator *op)
           }
           if (smooth_thickness) {
             /* thickness need to repeat process several times */
-            for (int r2 = 0; r2 < r * 20; r2++) {
+            for (int r2 = 0; r2 < 20; r2++) {
               BKE_gpencil_stroke_smooth_thickness(gps, i, factor);
             }
           }