GPencil: Add mode Merge to Simplify modifier

This option uses the same logic of the merge by distance but as an option of modifier to allow dynamic merge.

This option will be very useful for LANPR generated strokes.

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -1792,6 +1792,8 @@ class DATA_PT_gpencil_modifiers(ModifierButtonsPanel, Panel):
             col.prop(md, "factor")
         elif md.mode == 'SAMPLE':
             col.prop(md, "length")
+        elif md.mode == 'MERGE':
+            col.prop(md, "length", text="Threshold")
 
         col = layout.column()
         col.separator()