Renamed canvas_xlib to canvas_xrender

 - sgui_canvas_xlib in X11 backend is now known as sgui_canvas_xrender
   to avoid confusion with non xrender based implementation
 - xlib_pixap renamed to xrender_pixmap for same reason
 - sgui_canvas_xlib has a font cache localy, per canvas instance,
   allocated when needed
 - sgui_pixmap_

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added xlib based canvas as fallback to X11 backend

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