Introduce experimental G2D acceleration

This initial G2D support code can speed up moving windows in XFCE. Currently
disabled by default, but can be enabled by editing /etc/X11/xorg.conf and
adding the following line to the "Device" section:

        Option          "AccelMethod" "G2D"

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>