Boilerplate for the mirror modifier.

This was copied from the old modifier DNA/RNA, but isn't actually used
in the old modifier at all.

@@ -822,6 +822,7 @@ class NodeTreeMainUpdater {
           bNodeSocket &input = *node->input_sockets()[0];
           BLI_assert(input.is_available() && input.type == SOCK_MENU);
           this->set_enum_ptr(*input.default_value_typed<bNodeSocketValueMenu>(), enum_items);
+          enum_items->remove_user_and_delete_if_last();

@@ -151,6 +151,8 @@ class OBJECT_MT_modifier_add_generate(ModifierAddMenu, Menu):
             self.operator_modifier_add(layout, 'WIREFRAME')
         if ob_type == 'GREASEPENCIL':
             self.operator_modifier_add(layout, 'GREASE_PENCIL_SUBDIV')
+        if ob_type == 'GREASEPENCIL':

@@ -2670,0 +2672,4 @@
+  ModifierData modifier;
+  GreasePencilModifierInfluenceData influence;
+  struct Object *object;
+  /** Flags. */

@@ -0,0 +6,4 @@
+ * \ingroup modifiers
+ */
+
+#include "BLI_array_utils.hh"

@@ -0,0 +178,4 @@
+  }
+  else {
+    /* Create masked geometry, then mirror it. */
+    bke::AnonymousAttributePropagationInfo propagation_info;

@@ -179,3 +179,3 @@
   IDPropertyUIDataEnumItem *idprop_items = nullptr;
 
-  if (value->enum_items) {
+  if (value->enum_items && !value->enum_items->items.is_empty()) {

@@ -2670,0 +2678,4 @@
+} GreasePencilMirrorModifierData;
+
+typedef enum GreasePencilMirrorModifierFlag {
+  MOD_GREASE_PENCIL_MIRROR_CLIPPING = (1 << 0),

@@ -151,6 +151,8 @@ class OBJECT_MT_modifier_add_generate(ModifierAddMenu, Menu):
             self.operator_modifier_add(layout, 'WIREFRAME')
         if ob_type == 'GREASEPENCIL':
             self.operator_modifier_add(layout, 'GREASE_PENCIL_SUBDIV')
+        if ob_type == 'GREASEPENCIL':

@@ -0,0 +162,4 @@
+  const IndexMask curves_mask = modifier::greasepencil::get_filtered_stroke_mask(
+      ctx.object, src_curves, mmd.influence, curve_mask_memory);
+
+  bke::CurvesGeometry result;