Avoid initialization of graphical elements for a server.

Avoid allocating memory and resources for graphical elements that will never be drawn when starting a dedicated server. This reduces the server memory footprint by approx 17 MiB.

OpenRA.Game/Graphics/VoxelLoader.cs

@@ -52,7 +52,7 @@ namespace OpenRA.Graphics
 				if (allocated)
 					throw new SheetOverflowException("");
 				allocated = true;
-				return SheetBuilder.AllocateSheet(Game.Renderer.SheetSize);
+				return SheetBuilder.AllocateSheet(Game.Settings.Graphics.SheetSize);
 			};
 
 			return new SheetBuilder(SheetType.DualIndexed, allocate);