Mesh: simple extract mask optimizations

- Avoid per-vertex custom-data lookup
- Break once a face has a mask vertex.

source/blender/editors/mesh/editmesh_mask_extract.c

@@ -101,15 +101,18 @@ static int paint_mask_extract_exec(bContext *C, wmOperator *op)
   BMIter face_iter;
 
   /* Delete all unmasked faces */
+  const int cd_vert_mask_offset = CustomData_get_offset(&bm->vdata, CD_PAINT_MASK);
+  BLI_assert(cd_vert_mask_offset != -1);
   BM_mesh_elem_hflag_disable_all(bm, BM_VERT | BM_EDGE | BM_FACE, BM_ELEM_TAG, false);
 
   float mask_threshold = RNA_float_get(op->ptr, "mask_threshold");
   BM_ITER_MESH (f, &iter, bm, BM_FACES_OF_MESH) {
     bool keep_face = true;
     BM_ITER_ELEM (v, &face_iter, f, BM_VERTS_OF_FACE) {
-      float mask = BM_elem_float_data_get(&bm->vdata, v, CD_PAINT_MASK);
+      const float mask = BM_ELEM_CD_GET_FLOAT(v, cd_vert_mask_offset);
       if (mask < mask_threshold) {
         keep_face = false;
+        break;
       }
     }
     BM_elem_flag_set(f, BM_ELEM_TAG, !keep_face);