Revert "Run graphics rendering on a dedicated thread."

This reverts commit b9be52c5428d4a3862d62fe3a2c01663bd3692c3.

OpenRA.Platforms.Default/Sdl2Input.cs

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