compositor: Remove atom_x_root_pixmap

It's unused

https://bugzilla.gnome.org/show_bug.cgi?id=720106
Move _NET_WM_WINDOW_OPACITY handling to the standard window-props interface

This removes one X11 dependency that the MetaWindowActor interface has,
making it easier for us to use Wayland on this one...

https://bugzilla.gnome.org/show_bug.cgi?id=720106