fully support _GTK_FRAME_EXTENTS

fully support _GTK_FRAME_EXTENTS

gtk apps set an atom which provides information about the area
where non-window content (eg. shadows) may be drawn; this area
must not be used in placement calculations.

the easiest method for implementing this functionality was to add
a case to the compositor geometry interceptors which effectively
flip the client struct geometry values such that the E_Client->client
is outside of the more commonly used E_Client->x/y/w/h

fix T2744