Added thread-affinity checks to SDL2 renderer.

If a call is made into a graphics resource that has thread-affinity, from a thread other than the one that created the graphics device, an exception will now be thrown to make debugging easier.

OpenRA.Platforms.Default/ThreadAffine.cs

@@ -0,0 +1,31 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2015 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see COPYING.
+ */
+#endregion
+
+using System;
+using System.Threading;
+
+namespace OpenRA.Platforms.Default
+{
+	abstract class ThreadAffine
+	{
+		readonly int managedThreadId;
+
+		protected ThreadAffine()
+		{
+			managedThreadId = Thread.CurrentThread.ManagedThreadId;
+		}
+
+		protected void VerifyThreadAffinity()
+		{
+			if (managedThreadId != Thread.CurrentThread.ManagedThreadId)
+				throw new InvalidOperationException("Cross-thread operation not valid: This method must be called from the same thread that created this object.");
+		}
+	}
+}