Maim respects _NET_FRAME_EXTENTS now, as well as XShape clipping.

Mutter uses _NET_FRAME_EXTENTS to hide some graphical bugs, maim now respects that property.
Old maim screenshots in mutter

New maim screenshot in mutter

Maim now also respects the XShape extension, allowing shaped windows to be transparent.
Old xeyes screenshot

New xeyes screenshot