vcl: make OpenGLContext::makeCurrent unfeasibly faster.

Don't set the same context if it is already set. Apparently this is
something that is unbelievably costly on Windows, and very costly on
Linux. Instead check if the context is already set, and don't re-set it.

Change-Id: If4fed3e555e1388031446be76d6fdfde5b9f13e5