Remove System.Bitmap from map preview loading.

OpenRA.Game/Graphics/SheetBuilder.cs

@@ -12,6 +12,7 @@
 using System;
 using System.Collections.Generic;
 using System.Drawing;
+using OpenRA.FileFormats;
 
 namespace OpenRA.Graphics
 {
@@ -83,6 +84,14 @@ namespace OpenRA.Graphics
 			return rect;
 		}
 
+		public Sprite Add(Png src)
+		{
+			var rect = Allocate(new Size(src.Width, src.Height));
+			Util.FastCopyIntoSprite(rect, src);
+			current.CommitBufferedData();
+			return rect;
+		}
+
 		public Sprite Add(Size size, byte paletteIndex)
 		{
 			var data = new byte[size.Width * size.Height];