This avoids calling PreLoad needlessly on a thread change during offscreen
rendering and breaks up the endless recursion due to lastTID != newTID.

There is no reason to do that, now that the SetGLTextureDesc bug is
fixed. This avoids an infinite recursion because PreLoad calls
ActivateContext at some point.

Thanks to Mauricio Teixeira <mteixeira@webset.net> for the translation.