Cleanup: Rework sculpt_undo.cc for readability

* Avoid usage of `applied` in favor of helper methods to indicate
  undo and redo requirements
* Rename `bmesh_restore_begin` and `bmesh_restore_end` to avoid
  similarity with geometry methods
* Swap `geometry_original` and `geometry_modified` to match
  expected meaning

Pull Request: https://projects.blender.org/blender/blender/pulls/139244

source/blender/editors/sculpt_paint/sculpt_undo.cc

@@ -182,6 +182,10 @@ struct NodeGeometry {
 struct Node;
 
 struct StepData {
+ private:
+  bool applied_ = true;
+
+ public:
   /**
    * The type of data stored in this undo step. For historical reasons this is often set when the
    * first undo node is pushed.
@@ -236,8 +240,6 @@ struct StepData {
   /* Modified geometry is stored after the modification and is restored from when redoing. */
   NodeGeometry geometry_modified;
 
-  bool applied;
-
   Mutex nodes_mutex;
 
   /**
@@ -255,6 +257,22 @@ struct StepData {
   Vector<std::unique_ptr<Node>> nodes;
 
   size_t undo_size;
+
+  /** Whether processing code needs to handle the current data as an undo step. */
+  bool needs_undo() const
+  {
+    return applied_;
+  }
+
+  void tag_needs_undo()
+  {
+    applied_ = true;
+  }
+
+  void tag_needs_redo()
+  {
+    applied_ = false;
+  }
 };
 
 struct SculptUndoStep {
@@ -573,13 +591,13 @@ static bool restore_face_sets(Object &object,
 
 static void bmesh_restore_generic(StepData &step_data, Object &object, const SculptSession &ss)
 {
-  if (step_data.applied) {
+  if (step_data.needs_undo()) {
     BM_log_undo(ss.bm, ss.bm_log);
-    step_data.applied = false;
+    step_data.tag_needs_redo();
   }
   else {
     BM_log_redo(ss.bm, ss.bm_log);
-    step_data.applied = true;
+    step_data.tag_needs_undo();
   }
 
   if (step_data.type == Type::Mask) {
@@ -618,42 +636,42 @@ static void bmesh_enable(Object &object, const StepData &step_data)
   ss.bm_log = BM_log_from_existing_entries_create(ss.bm, step_data.bmesh.bm_entry);
 }
 
-static void bmesh_restore_begin(bContext *C,
-                                StepData &step_data,
-                                Object &object,
-                                const SculptSession &ss)
+static void bmesh_handle_dyntopo_begin(bContext *C,
+                                       StepData &step_data,
+                                       Object &object,
+                                       const SculptSession &ss)
 {
-  if (step_data.applied) {
+  if (step_data.needs_undo()) {
     dyntopo::disable(C, &step_data);
-    step_data.applied = false;
+    step_data.tag_needs_redo();
   }
-  else {
+  else /* needs_redo */ {
     bmesh_enable(object, step_data);
 
     /* Restore the mesh from the first log entry. */
     BM_log_redo(ss.bm, ss.bm_log);
 
-    step_data.applied = true;
+    step_data.tag_needs_undo();
   }
 }
 
-static void bmesh_restore_end(bContext *C,
-                              StepData &step_data,
-                              Object &object,
-                              const SculptSession &ss)
+static void bmesh_handle_dyntopo_end(bContext *C,
+                                     StepData &step_data,
+                                     Object &object,
+                                     const SculptSession &ss)
 {
-  if (step_data.applied) {
+  if (step_data.needs_undo()) {
     bmesh_enable(object, step_data);
 
     /* Restore the mesh from the last log entry. */
     BM_log_undo(ss.bm, ss.bm_log);
 
-    step_data.applied = false;
+    step_data.tag_needs_redo();
   }
-  else {
+  else /* needs_redo */ {
     /* Disable dynamic topology sculpting. */
     dyntopo::disable(C, nullptr);
-    step_data.applied = true;
+    step_data.tag_needs_undo();
   }
 }
 
@@ -726,13 +744,13 @@ static void restore_geometry(StepData &step_data, Object &object)
 
   Mesh *mesh = static_cast<Mesh *>(object.data);
 
-  if (step_data.applied) {
-    restore_geometry_data(&step_data.geometry_modified, mesh);
-    step_data.applied = false;
+  if (step_data.needs_undo()) {
+    restore_geometry_data(&step_data.geometry_original, mesh);
+    step_data.tag_needs_redo();
   }
   else {
-    restore_geometry_data(&step_data.geometry_original, mesh);
-    step_data.applied = true;
+    restore_geometry_data(&step_data.geometry_modified, mesh);
+    step_data.tag_needs_undo();
   }
 }
 
@@ -749,12 +767,12 @@ static int bmesh_restore(bContext *C,
   switch (step_data.type) {
     case Type::DyntopoBegin:
       BKE_sculpt_update_object_for_edit(&depsgraph, &object, false);
-      bmesh_restore_begin(C, step_data, object, ss);
+      bmesh_handle_dyntopo_begin(C, step_data, object, ss);
       return true;
 
     case Type::DyntopoEnd:
       BKE_sculpt_update_object_for_edit(&depsgraph, &object, false);
-      bmesh_restore_end(C, step_data, object, ss);
+      bmesh_handle_dyntopo_end(C, step_data, object, ss);
       return true;
     default:
       if (ss.bm_log) {
@@ -1286,8 +1304,6 @@ static void geometry_push(const Object &object)
 
   step_data->type = Type::Geometry;
 
-  step_data->applied = false;
-
   NodeGeometry *geometry = geometry_get(*step_data);
   store_geometry_data(geometry, object);
 }
@@ -1456,7 +1472,6 @@ BLI_NOINLINE static void bmesh_push(const Object &object,
     unode = step_data->nodes.last().get();
 
     step_data->type = type;
-    step_data->applied = true;
 
     if (type == Type::DyntopoEnd) {
       step_data->bmesh.bm_entry = BM_log_entry_add(ss.bm_log);