use byte[] backing for Sheet. bitmap backing is still available for Chrome RGBA hack.

OpenRA.Game/Graphics/SpriteSheetBuilder.cs

@@ -17,7 +17,6 @@ namespace OpenRA.Graphics
 	{
 		public static void Initialize( TileSet tileset )
 		{
-			/* .tem: hack to allow incomplete theaters (interior) to work, falling back to temperate for the missing art */
 			exts = tileset.Extensions;
 			sprites = new Cache<string, Sprite[]>( LoadSprites );
 		}