Refactor: Move and replace Face Set SCULPT_stroke_is_first_brush_step

* Shift call to the Draw Face Set brush implementation as it is
  irrelevant for other brushes.
* Instead of checking for the first brush step, check to see if the
  value is still `SCULPT_FACE_SET_NONE`.

Pull Request: https://projects.blender.org/blender/blender/pulls/138949

source/blender/editors/sculpt_paint/brushes/draw_face_sets.cc

@@ -20,6 +20,8 @@
 #include "editors/sculpt_paint/sculpt_intern.hh"
 #include "editors/sculpt_paint/sculpt_undo.hh"
 
+#include "ED_sculpt.hh"
+
 #include "bmesh.hh"
 
 namespace blender::ed::sculpt_paint::brushes {
@@ -430,6 +432,17 @@ void do_draw_face_sets_brush(const Depsgraph &depsgraph,
 {
   const Brush &brush = *BKE_paint_brush_for_read(&sd.paint);
 
+  if (object.sculpt->cache->paint_face_set == SCULPT_FACE_SET_NONE) {
+    if (object.sculpt->cache->invert) {
+      /* When inverting the brush, pick the paint face mask ID from the mesh. */
+      object.sculpt->cache->paint_face_set = face_set::active_face_set_get(object);
+    }
+    else {
+      /* By default, create a new Face Sets. */
+      object.sculpt->cache->paint_face_set = face_set::find_next_available_id(object);
+    }
+  }
+
   switch (bke::object::pbvh_get(object)->type()) {
     case bke::pbvh::Type::Mesh:
       do_draw_face_sets_brush_mesh(depsgraph, object, brush, node_mask);

source/blender/editors/sculpt_paint/sculpt.cc

@@ -3217,23 +3217,6 @@ static void do_brush_action(const Depsgraph &depsgraph,
       depsgraph, ob, brush, memory);
   const IndexMask node_mask = cursor_sample_result.node_mask;
 
-  /* Draw Face Sets in draw mode makes a single undo push, in alt-smooth mode deforms the
-   * vertices and uses regular coords undo. */
-  /* It also assigns the paint_face_set here as it needs to be done regardless of the stroke type
-   * and the number of nodes under the brush influence. */
-  if (brush.sculpt_brush_type == SCULPT_BRUSH_TYPE_DRAW_FACE_SETS &&
-      SCULPT_stroke_is_first_brush_step(*ss.cache) && !ss.cache->alt_smooth)
-  {
-    if (ss.cache->invert) {
-      /* When inverting the brush, pick the paint face mask ID from the mesh. */
-      ss.cache->paint_face_set = face_set::active_face_set_get(ob);
-    }
-    else {
-      /* By default create a new Face Sets. */
-      ss.cache->paint_face_set = face_set::find_next_available_id(ob);
-    }
-  }
-
   /* Only act if some verts are inside the brush area. */
   if (node_mask.is_empty()) {
     return;

source/blender/editors/sculpt_paint/sculpt_intern.hh

@@ -252,7 +252,7 @@ struct StrokeCache {
   SculptRakeData rake_data;
 
   /* The face set being painted. */
-  int paint_face_set = 0;
+  int paint_face_set = SCULPT_FACE_SET_NONE;
 
   /**
    * Symmetry index between 0 and 7 bit combo.