$ gnome-screensaver-preferences 
** Message: Found best visual for GL: 0x26
Error: Cairo 1.4.2 does not yet support the requested image format:
        Depth: 16
        Alpha mask: 0x00000000
        Red   mask: 0x0000003f
        Green mask: 0x000007c0
        Blue  mask: 0x0000f800
Please file an enhancement request (quoting the above) at:
http://bugs.freedesktop.org/enter_bug.cgi?product=cairo
gnome-screensaver-preferences: /build/buildd/libcairo-1.4.2/src/cairo-image-surface.c:199: _cairo_format_from_pixman_format: Assertion `NOT_REACHED' failed.
Aborted (core dumped)