Fix: Performance regression when using bevel and subdivision modifier

PR https://projects.blender.org/blender/blender/pulls/144233.
Fix #142296 performance regression caused by merging UV positions
based on face area weights, replacing it with mean average weights
computation. The former method resulted in higher probability in making
subdivision cache invalid between frame updates and thus causing expensive
recreation of blender::bke::subdiv::OpenSubdiv_Evaluator object.
While this fix isn't directly answering a question why specific UV position
updates would cause this reevaluation of subdivision object, it fixes
the performance regression caused by #139595 PR.

source/blender/bmesh/tools/bmesh_bevel.cc

@@ -814,24 +814,19 @@ static void bevel_merge_uvs(BevelParams *bp, BMesh *bm)
     int uv_data_offset = CustomData_get_n_offset(&bm->ldata, CD_PROP_FLOAT2, i);
     for (Vector<UVVertBucket> &uv_vert_buckets : bp->uv_vert_maps[i].values()) {
       for (UVVertBucket &uv_vert_bucket : uv_vert_buckets) {
-        /* Using face weights instead of mean average because it produces slightly better results,
-         * although this is purely empirical and subjective. */
-        float weight_sum = 0.0f;
+        int num_uv_verts = uv_vert_bucket.size();
+        if (num_uv_verts <= 1) {
+          continue;
+        }
         float uv[2] = {0.0f, 0.0f};
         for (BMLoop *l : uv_vert_bucket) {
           float *luv = BM_ELEM_CD_GET_FLOAT_P(l, uv_data_offset);
-          float weighted_luv[2] = {0.0f, 0.0f};
-          float face_area = BM_face_calc_area(l->f);
-          mul_v2_v2fl(weighted_luv, luv, face_area);
-          add_v2_v2(uv, weighted_luv);
-          weight_sum += face_area;
+          add_v2_v2(uv, luv);
         }
-        if (uv_vert_bucket.size() > 1 && weight_sum > 0.0f) {
-          mul_v2_fl(uv, 1.0f / weight_sum);
-          for (BMLoop *l : uv_vert_bucket) {
-            float *luv = BM_ELEM_CD_GET_FLOAT_P(l, uv_data_offset);
-            copy_v2_v2(luv, uv);
-          }
+        mul_v2_fl(uv, 1.0f / (float)num_uv_verts);
+        for (BMLoop *l : uv_vert_bucket) {
+          float *luv = BM_ELEM_CD_GET_FLOAT_P(l, uv_data_offset);
+          copy_v2_v2(luv, uv);
         }
       }
     }