GC fix

ScriptableRenderPipeline/Core/CoreRP/Volume/VolumeManager.cs

         const int k_MaxLayerCount = 32;
 
         // Cached lists of all volumes (sorted by priority) by layer mask
-        readonly Dictionary<LayerMask, List<Volume>> m_SortedVolumes;
+        readonly Dictionary<int, List<Volume>> m_SortedVolumes;
-        readonly Dictionary<LayerMask, bool> m_SortNeeded;
+        readonly Dictionary<int, bool> m_SortNeeded;
 
         // Internal list of default state for each component type - this is used to reset component
         // states on update instead of having to implement a Reset method on all components (which
 
         VolumeManager()
         {
-            m_SortedVolumes = new Dictionary<LayerMask, List<Volume>>();
+            m_SortedVolumes = new Dictionary<int, List<Volume>>();
-            m_SortNeeded = new Dictionary<LayerMask, bool>();
+            m_SortNeeded = new Dictionary<int, bool>();
             m_TempColliders = new List<Collider>(8);
             m_ComponentsDefaultState = new List<VolumeComponent>();