D1886: GPencil - Add smooth iterations parameter to get better quality

After some test, a new iteration parameter has been added in order to
apply repetitive smoothing to the stroke. By default 1 iteration is applied,
but can used any number between 1 and 3.

The repetition uses different levels of intensity from 100% of the defined smooth
factor for the first loop, 50% for the second and 25% for the third. We use in each
loop a smaller value in order to avoid deform too much the stroke.

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -629,6 +629,8 @@ class GreasePencilDataPanel:
         col = layout.column(align=True)
         col.label(text="New Stroke Quality:")
         col.prop(gpl, "pen_smooth_factor")
+        col.prop(gpl, "pen_smooth_steps")
+        col.separator()
         col.prop(gpl, "pen_subdivision_steps")