gtkwindow: Fix resize without "_GTK_FRAME_EXTENTS"

gtkwindow: Fix resize without "_GTK_FRAME_EXTENTS"

git commit a5b1cdd0 introduced a regression where CSD windows are not
resizable with metacity.

Reason being that metacity does not support "_GTK_FRAME_EXTENTS" and
therefore gtk_window_supports_client_shadow() would always return FALSE.

This explains why it works with window managers which support
"_GTK_FRAME_EXTENTS" such as mutter/gnome-shell or xfwm4.

Partially revert commit a5b1cdd0 to reinstate the logic in
get_shadow_width().

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=757805