Add a visualisation mode for depth sprites.

For now this displays the raw sprites.  It will
eventually be repurposed for rendering the proper
depth data.

OpenRA.Game/Graphics/WorldRenderer.cs

@@ -132,6 +132,9 @@ namespace OpenRA.Graphics
 			if (World.WorldActor.Disposed)
 				return;
 
+			if (devTrait.Value != null)
+				Game.Renderer.WorldSpriteRenderer.SetDepthPreviewEnabled(devTrait.Value.ShowDepthPreview);
+
 			RefreshPalette();
 
 			if (World.Type == WorldType.Shellmap && !Game.Settings.Game.ShowShellmap)