Remove some unrequired Sheet.CreateBuffer or Sheet.ReleaseBuffer calls.

- In CursorManager, we can release the buffer on the final sheet after loading cursors. We don't need to release the buffer on every sheet in the builder, as the builder will handle that.
- In SpriteCache, we don't need to call CreateBuffer explicitly, as the builder will do that for us.
- In RadarWidget, we don't need to call CreateBuffer explicitly, as GetData will do that for us.

OpenRA.Game/Graphics/CursorManager.cs

@@ -91,10 +91,6 @@ namespace OpenRA.Graphics
 			}
 
 			CreateOrUpdateHardwareCursors();
-
-			foreach (var s in sheetBuilder.AllSheets)
-				s.ReleaseBuffer();
-
 			Update();
 		}
 
@@ -128,6 +124,8 @@ namespace OpenRA.Graphics
 				}
 			}
 
+			sheetBuilder.Current.ReleaseBuffer();
+
 			hardwareCursorsDoubled = graphicSettings.CursorDouble;
 		}