Run graphics rendering on a dedicated thread.

The main game thread can offload some of the CPU cost to the rendering thread, freeing up its time to run more logic and render ticks.

OpenRA.Platforms.Default/Sdl2Input.cs

@@ -191,8 +191,6 @@ namespace OpenRA.Platforms.Default
 				inputHandler.OnMouseInput(pendingMotion.Value);
 				pendingMotion = null;
 			}
-
-			OpenGL.CheckGLError();
 		}
 	}
 }