Visual_t     GetVisual() const;
   Int_t        GetScreen() const;
   Int_t        GetDepth() const;
   Colormap_t   GetColormap() const;
these methods are usefull in some cases where direct X11 manipulation outside
of TVirtualX is needed. Also added code in TGX11 to detect the best X11
visual available. Some X servers insist on using by default an 8 bit
PseudoColor visual while 16 or 24 bit TrueColor (or DirectColor) visual are
available. Using the best visual is in particular important for image
processing classes.


git-svn-id: http://root.cern.ch/svn/root/trunk@4048 27541ba8-7e3a-0410-8455-c3a389f83636