Support _NET_FRAME_EXTENTS

[9ary]

Some window managers, such as Mutter (GNOME 3), use a decoration window larger than the actual border to draw things like shadows. Interpreting this hint properly allows proper cropping of that area. See this for an example.
@expectocode suggests that shadows might be desirable to have in screenshots, á-la macOS, so a command-line switch is preferred.

I'm personally running i3 and always removing the decorations when taking screenshots, so I'm not really interested in implementing this myself, and I don't want to install GNOME to test it, so I'm leaving this one up for grabs.