xwm: use last focused window for guessing transient parent

On X the global absolute coordinates are sent in ConfigureNotify and transient
windows are mapped exactly on that position. On Wayland we don't have the
concept of global coordinates, and that's a problem for transient surfaces
without transient_for set because they rely on such hint for setting their
positioning.

So this solution is a workaround. It guesses a parent based on the last
focused window to determine the relative position of the transient surface.
This put transient windows of Chrome browser back to work.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
diff --git a/src/xwayland/xwayland.h b/src/xwayland/xwayland.h
index 7e1ea45..23c6a77 100644
--- a/src/xwayland/xwayland.h
+++ b/src/xwayland/xwayland.h
@@ -57,6 +57,7 @@
 	struct weston_xserver *server;
 	xcb_window_t wm_window;
 	struct weston_wm_window *focus_window;
+	struct weston_wm_window *focus_latest;
 	struct theme *theme;
 	xcb_cursor_t *cursors;
 	int last_cursor;