Paint brush no long SCULPT_undo_push_node per dab (except for first

stroke) for dyntopo.  Instead it updates the original vertex color
customdata layer.

Calling into the undo system destroys threading with dyntopo, as its
undo code is single-threaded.

source/blender/blenkernel/BKE_pbvh.h

@@ -229,6 +229,7 @@ void BKE_pbvh_free(PBVH *pbvh);
 /** update original data, only data whose r_** parameters are passed in will be updated*/
 void BKE_pbvh_bmesh_update_origvert(
     PBVH *pbvh, struct BMVert *v, float **r_co, float **r_no, float **r_color);
+void BKE_pbvh_update_origcolor_bmesh(PBVH *pbvh, PBVHNode *node);
 
 /* Hierarchical Search in the BVH, two methods:
  * - for each hit calling a callback

source/blender/blenkernel/intern/pbvh.c

@@ -1424,6 +1424,30 @@ static int pbvh_flush_bb(PBVH *pbvh, PBVHNode *node, int flag)
   return update;
 }
 
+void BKE_pbvh_update_origcolor_bmesh(PBVH *pbvh, PBVHNode *node)
+{
+  PBVHVertexIter vd;
+
+  if (!pbvh->bm || !pbvh->cd_origvcol_offset) {
+    return;
+  }
+
+  int cd_vcol_offset = CustomData_get_offset(&pbvh->bm->vdata, CD_PROP_COLOR);
+
+  if (cd_vcol_offset == -1) {
+    return;
+  }
+
+  BKE_pbvh_vertex_iter_begin(pbvh, node, vd, PBVH_ITER_ALL)
+  {
+    float *c1 = BM_ELEM_CD_GET_VOID_P(vd.bm_vert, pbvh->cd_origvcol_offset);
+    float *c2 = BM_ELEM_CD_GET_VOID_P(vd.bm_vert, cd_vcol_offset);
+
+    copy_v4_v4(c1, c2);
+  }
+  BKE_pbvh_vertex_iter_end;
+}
+
 void BKE_pbvh_update_bounds(PBVH *pbvh, int flag)
 {
   if (!pbvh->nodes) {

source/blender/editors/sculpt_paint/sculpt.c

@@ -5768,7 +5768,15 @@ static void do_brush_action_task_cb(void *__restrict userdata,
     BKE_pbvh_node_mark_update_mask(data->nodes[n]);
   }
   else if (ELEM(data->brush->sculpt_tool, SCULPT_TOOL_PAINT, SCULPT_TOOL_SMEAR)) {
-    SCULPT_undo_push_node(data->ob, data->nodes[n], SCULPT_UNDO_COLOR);
+    //make sure we have at least one undo_color node
+    if (!ss->bm || SCULPT_stroke_is_first_brush_step(ss->cache)) {
+      SCULPT_undo_push_node(data->ob, data->nodes[n], SCULPT_UNDO_COLOR);
+    }
+
+    if (ss->bm) {
+      BKE_pbvh_update_origcolor_bmesh(ss->pbvh, data->nodes[n]);
+    }
+
     BKE_pbvh_node_mark_update_color(data->nodes[n]);
   }
   else {