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/SpriteCache.cs

@@ -80,9 +80,6 @@ namespace OpenRA.Graphics
 
 		public void LoadReservations(ModData modData)
 		{
-			foreach (var sb in SheetBuilders.Values)
-				sb.Current.CreateBuffer();
-
 			var pendingResolve = new List<(
 				string Filename,
 				int FrameIndex,