surface: Store popup parent

Store popup parents separately from transient-for parents,
since these are separate concepts with different behaviors.

gdk/broadway/gdksurface-broadway.c

@@ -205,6 +205,7 @@ _gdk_broadway_display_create_surface (GdkDisplay     *display,
   g_object_unref (frame_clock);
 
   surface->surface_type = surface_type;
+  surface->parent = parent;
   surface->x = x;
   surface->y = y;
   surface->width = width;

gdk/gdksurfaceprivate.h

@@ -32,7 +32,8 @@ struct _GdkSurface
 
   GdkDisplay *display;
 
-  GdkSurface *transient_for;
+  GdkSurface *transient_for; /* for toplevels */
+  GdkSurface *parent;        /* for popups */
 
   gpointer widget;
 

gdk/wayland/gdksurface-wayland.c

@@ -535,6 +535,7 @@ _gdk_wayland_display_create_surface (GdkDisplay     *display,
     }
 
   surface->surface_type = surface_type;
+  surface->parent = parent;
   surface->x = x;
   surface->y = y;
   surface->width = width;

gdk/x11/gdksurface-x11.c

@@ -811,6 +811,7 @@ _gdk_x11_display_create_surface (GdkDisplay     *display,
   g_object_unref (frame_clock);
 
   surface->surface_type = surface_type;
+  surface->parent = parent;
   surface->x = x;
   surface->y = y;
   surface->width = width;