Made SheetBuilder rely on global settings rather than global renderer.

Additionally exposed an InitializeSettings method on game to initialize the global settings so that other classes can set up all the secret dependencies on the global settings required.

OpenRA.Game/Graphics/SheetBuilder.cs

@@ -46,7 +46,7 @@ namespace OpenRA.Graphics
 		}
 
 		public SheetBuilder(SheetType t)
-			: this(t, Game.Renderer.SheetSize) { }
+			: this(t, Game.Settings.Graphics.SheetSize) { }
 
 		public SheetBuilder(SheetType t, int sheetSize)
 			: this(t, () => AllocateSheet(sheetSize)) { }