Remove event handlers to CellEntryChanged when done.

Several classes would attach event handlers to the Map which would live longer then they did. Detaching them when no longer needed allows the GC to reclaim them.

OpenRA.Mods.Common/Traits/World/EditorResourceLayer.cs

@@ -207,6 +207,8 @@ namespace OpenRA.Mods.Common.Traits
 			foreach (var kv in spriteLayers.Values)
 				kv.Dispose();
 
+			Map.MapResources.Value.CellEntryChanged -= UpdateCell;
+
 			disposed = true;
 		}
 	}

OpenRA.Mods.Common/Traits/World/ResourceLayer.cs

@@ -294,6 +294,8 @@ namespace OpenRA.Mods.Common.Traits
 			foreach (var kv in spriteLayers.Values)
 				kv.Dispose();
 
+			RenderContent.CellEntryChanged -= UpdateSpriteLayers;
+
 			disposed = true;
 		}