Removed unneded glx window in Xlib backend

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Minor simplifications in Xlib backend

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Removed canvas background color

 - Canvas has fully transparent background
 - Canvas does not know window background color and has no
   such attribute
 - Window has background color set and blends canvas ontop of it

Note:
Memory pixmap/canvas probably has a mixup between premultiplied alpha
and normal blending somewhere.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>