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/Sdl2Input.cs

@@ -15,7 +15,7 @@ using SDL2;
 
 namespace OpenRA.Platforms.Default
 {
-	public class Sdl2Input
+	class Sdl2Input
 	{
 		MouseButton lastButtonBits = (MouseButton)0;