Fix render issues with main preview widget

 - I seem to have fixed ths issues with the main preview widget.  It
   seems you just need to set the right window attributes to stop it from
   breaking.  Though when opengl is enabled, there appears to be a weird
   background glitch in the Qt stuff -- I'm not entirely sure what's
   going on.  Bug in Qt?

   Also fixed the layout issues, and the widget now properly resizes and
   centers in to its parent widget.

 - Prevent the render loop from accessing data if the data isn't valid.
   Because obs->data is freed before the graphics stuff, it can cause
   the graphics to keep trying to query the obs->data.displays_mutex
   after it had already been destroyed.