wayland: Handle subsurface as popup parent

wayland: Handle subsurface as popup parent

When a subsurface is used as a parent of a popup, GDK needs to traverse
up to the transient-for as the next parent, to properly find the parent
used by the popup positioner. This is because the parent of a popup
must always either be an xdg_popup or an xdg_surface, but traversing
the "parent" (in GDK terms) upwards from a subsurface will end up on
the fake root window before we hit the actual parent (in Wayland terms).

https://bugzilla.gnome.org/show_bug.cgi?id=776225