Maintain APV cells loaded if at least one scene still references that…

… cell (#5900)

* Add override checkbox.

* Fix for problem when max subdiv is smaller than index voxel update size.

* Revert "Merge branch 'HDRP/add-override-checkbox-for-pv' into HDRP/investigate-issue-with-faulty-index"

This reverts commit e33a421, reversing
changes made to 924763f.

com.unity.render-pipelines.core/Runtime/Lighting/ProbeVolume/ProbeReferenceVolume.cs

@@ -545,6 +545,15 @@ public struct ExtraDataActionInput
         // List of info for cells that are yet to be loaded.
         private List<CellSortInfo> m_CellsToBeLoaded = new List<CellSortInfo>();
 
+
+        // Ref counting here as a separate dictionary as a temporary measure to facilitate future changes that will soon go in.
+        // cell.index, refCount
+        Dictionary<int, int> m_CellRefCounting = new Dictionary<int, int>();
+        void InvalidateAllCellRefs()
+        {
+            m_CellRefCounting.Clear();
+        }
+
         bool m_NeedLoadAsset = false;
         bool m_ProbeReferenceVolumeInit = false;
         bool m_EnabledBySRP = false;
@@ -693,20 +702,34 @@ void RemoveCell(Cell cell)
         {
             if (cell.loaded)
             {
-                if (cells.ContainsKey(cell.index))
-                    cells.Remove(cell.index);
+                bool needsUnloading = true;
+                if (m_CellRefCounting.ContainsKey(cell.index))
+                {
+                    m_CellRefCounting[cell.index]--;
+                    needsUnloading = m_CellRefCounting[cell.index] <= 0;
+                    if (needsUnloading)
+                    {
+                        m_CellRefCounting[cell.index] = 0;
+                    }
+                }
 
-                if (m_ChunkInfo.ContainsKey(cell.index))
-                    m_ChunkInfo.Remove(cell.index);
+                if (needsUnloading)
+                {
+                    if (cells.ContainsKey(cell.index))
+                        cells.Remove(cell.index);
 
-                if (cell.flatIdxInCellIndices >= 0)
-                    m_CellIndices.MarkCellAsUnloaded(cell.flatIdxInCellIndices);
+                    if (m_ChunkInfo.ContainsKey(cell.index))
+                        m_ChunkInfo.Remove(cell.index);
 
-                RegId cellBricksID = new RegId();
-                if (m_CellToBricks.TryGetValue(cell, out cellBricksID))
-                {
-                    ReleaseBricks(cellBricksID);
-                    m_CellToBricks.Remove(cell);
+                    if (cell.flatIdxInCellIndices >= 0)
+                        m_CellIndices.MarkCellAsUnloaded(cell.flatIdxInCellIndices);
+
+                    RegId cellBricksID = new RegId();
+                    if (m_CellToBricks.TryGetValue(cell, out cellBricksID))
+                    {
+                        ReleaseBricks(cellBricksID);
+                        m_CellToBricks.Remove(cell);
+                    }
                 }
             }
 
@@ -715,6 +738,9 @@ void RemoveCell(Cell cell)
 
         void AddCell(Cell cell, List<Chunk> chunks)
         {
+            if (m_CellRefCounting.ContainsKey(cell.index)) m_CellRefCounting[cell.index]++;
+            else m_CellRefCounting.Add(cell.index, 1);
+
             cell.loaded = true;
             cells[cell.index] = cell;
 
@@ -898,6 +924,9 @@ void PerformPendingLoading()
             // Load the ones that are already active but reload if we said we need to load
             if (m_HasChangedIndex)
             {
+                // We changed index so all assets are going to be re-loaded, hence the refs will be repopulated from scratch
+                InvalidateAllCellRefs();
+
                 foreach (var asset in m_ActiveAssets.Values)
                 {
                     LoadAsset(asset);