attr/m49044: conditionally merge bottom two scalable LoDs

Due to early termination of the octree, it is not guaranteed that
the number of nodes per level monotonically increases with depth.
Merging the two finest LoDs when scalable LoD generation is enabled
attempts to restore this property and match the behaviour of the
distance based LoD subsampler.

tmc3/PCCTMC3Common.h

@@ -957,7 +957,15 @@ buildPredictorsFast(
   int32_t num_detail_levels = aps.num_detail_levels;
   if (aps.scalable_lifting_enabled_flag == 1) {
     // NB: when partial decoding is enabled, LoDs correspond to octree levels
-    num_detail_levels = INT_MAX;
+    num_detail_levels = std::numeric_limits<int>::max();
+    ;
+  }
+
+  // prepare temporal buffer
+  std::vector<uint32_t> indexesOfSbsample;
+  indexesOfSbsample.resize(0);
+  if (aps.scalable_lifting_enabled_flag) {
+    indexesOfSbsample.reserve(pointCount);
   }
 
   std::vector<Box3<int32_t>> bBoxes;
@@ -974,6 +982,26 @@ buildPredictorsFast(
         aps, pointCloud, packedVoxel, input, lodIndex, retained, indexes);
     }
     const int32_t endIndex = indexes.size();
+    if (aps.scalable_lifting_enabled_flag) {
+      indexesOfSbsample.resize(endIndex);
+      if (lodIndex == 0) {
+        for (int32_t i = startIndex; i < endIndex; i++) {
+          indexesOfSbsample[i] = indexes[i];
+        }
+      }
+      if (lodIndex == 1) {
+        if (endIndex - startIndex > startIndex) {
+          for (int32_t i = 0; i < startIndex; i++) {
+            indexes[i] = indexesOfSbsample[i];
+          }
+          predIndex = int32_t(pointCount);
+          computeNearestNeighbors(
+            aps, pointCloud, packedVoxel, retained, 0, startIndex,
+            lodIndex - 1, indexes, predictors, pointIndexToPredictorIndex,
+            predIndex, bBoxes);
+        }
+      }
+    }
 
     computeNearestNeighbors(
       aps, pointCloud, packedVoxel, retained, startIndex, endIndex, lodIndex,