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.

source/blender/makesrna/intern/rna_gpencil_modifier.c

@@ -622,6 +622,11 @@ static void rna_def_modifier_gpencilsimplify(BlenderRNA *brna)
        ICON_IPO_EASE_IN_OUT,
        "Sample",
        "Sample a curve using a fixed length"},
+      {GP_SIMPLIFY_MERGE,
+       "MERGE",
+       ICON_IPO_EASE_IN_OUT,
+       "Merge",
+       "Sample a curve using doing merge of vertex"},
       {0, NULL, 0, NULL, NULL},
   };