Allow empty sprites to be allocated.

OpenRA.Game/Graphics/SheetBuilder.cs

@@ -55,6 +55,10 @@ namespace OpenRA.Graphics
 		public Sprite Add(byte[] src, Size size) { return Add(src, size, float2.Zero); }
 		public Sprite Add(byte[] src, Size size, float2 spriteOffset)
 		{
+			// Don't bother allocating empty sprites
+			if (size.Width == 0 || size.Height == 0)
+				return new Sprite(current, Rectangle.Empty, spriteOffset, channel, BlendMode.Alpha);
+
 			var rect = Allocate(size, spriteOffset);
 			Util.FastCopyIntoChannel(rect, src);
 			current.CommitData();