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.Mods.Common/Widgets/RadarWidget.cs

@@ -97,7 +97,6 @@ namespace OpenRA.Mods.Common.Widgets
 
 			// The four layers are stored in a 2x2 grid within a single texture
 			radarSheet = new Sheet(SheetType.BGRA, new Size(2 * previewWidth, 2 * previewHeight).NextPowerOf2());
-			radarSheet.CreateBuffer();
 			radarData = radarSheet.GetData();
 
 			MapBoundsChanged();