Implements the "prior window window geometry dimensions" as described in
the documentation of 'xdg_toplevel' request 'unset_maximized':
"If available and applicable, the compositor will include the window
geometry dimensions the window had prior to being maximized in the
configure event."
and 'unset_fullscreen':
"The compositor may include the previous window geometry dimensions in
the configure event, if applicable."
Fixes https://gitlab.gnome.org/GNOME/mutter/issues/792.
a93ef84d57