GtkTooltip: fix tooltip position when inside a Popover in Wayland

When pointer is over a Popover in Wayland, gdk_window_get_device_position()
call fails to retrieve x relative to the passed in toplevel widget,
and instead returns x relative to the Popover. So to fix that behaviour
in the GtkTooltip code, we use gtk_widget_translate_coordinates()
to obtain the x offset of the Popover wrt the toplevel widget, and then
add that to the previous incomplete pointer_x so now pointer_x contains
the expected x relative to the 'effective_toplevel' widget.

This fixes issue #1708
7 jobs for BUG_tooltip_position_CLEAN in 43 minutes and 40 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #947680
debian-meson

00:09:02

passed #947677
fedora-autotools

00:19:01

manual #947678
allowed to fail manual
fedora-distcheck
passed #947679
fedora-meson

00:18:18

manual #947682
win32-ps allowed to fail manual
msys2-mingw64-autotools
passed #947681
win32-ps
msys2-mingw64-meson

00:04:02

 
  Docs
passed #947683
reference

00:24:39