Added an extension method to clone bitmaps with a 32bbpArgb pixel format.

OpenRA.Game/Exts.cs

@@ -11,6 +11,7 @@
 using System;
 using System.Collections.Generic;
 using System.Drawing;
+using System.Drawing.Imaging;
 using System.Globalization;
 using System.Linq;
 using System.Reflection;
@@ -434,6 +435,25 @@ namespace OpenRA
 
 		public static Rectangle Bounds(this Bitmap b) { return new Rectangle(0, 0, b.Width, b.Height); }
 
+		public static Bitmap CloneWith32bbpArgbPixelFormat(this Bitmap original)
+		{
+			// Note: We would use original.Clone(original.Bounds(), PixelFormat.Format32bppArgb)
+			// but this doesn't work on mono.
+			var clone = new Bitmap(original.Width, original.Height, PixelFormat.Format32bppArgb);
+			try
+			{
+				using (var g = System.Drawing.Graphics.FromImage(clone))
+					g.DrawImage(original, original.Bounds());
+			}
+			catch (Exception)
+			{
+				clone.Dispose();
+				throw;
+			}
+
+			return clone;
+		}
+
 		public static int ToBits(this IEnumerable<bool> bits)
 		{
 			var i = 0;

OpenRA.Game/Map/Map.cs

@@ -398,7 +398,7 @@ namespace OpenRA
 				if (CustomPreview.PixelFormat != PixelFormat.Format32bppArgb)
 				{
 					var original = CustomPreview;
-					CustomPreview = original.Clone(original.Bounds(), PixelFormat.Format32bppArgb);
+					CustomPreview = original.CloneWith32bbpArgbPixelFormat();
 					original.Dispose();
 				}
 			}

OpenRA.Game/Map/MapPreview.cs

@@ -159,7 +159,7 @@ namespace OpenRA
 						if (CustomPreview.PixelFormat != PixelFormat.Format32bppArgb)
 						{
 							var original = CustomPreview;
-							CustomPreview = original.Clone(original.Bounds(), PixelFormat.Format32bppArgb);
+							CustomPreview = original.CloneWith32bbpArgbPixelFormat();
 							original.Dispose();
 						}
 					}