This should give us more flexibility for buffer size vs surface
size.
Unfortunately, mutter doesn't play along currently, so this is
only useful for kwin, weston or sway.
When grabbing the seat for an xdg popup using xdg_popup_grab() in response
to a touch-end event, we request the grab a little late and the touch is no
longer being tracked by gdkseat. This means that
_gdk_wayland_seat_get_last_implicit_grab_serial() right now can not provide
us with the serial for that touchpoint, because that serial was stored on
the GdkWaylandTouchData that is already gone.
To still provide the compositor a valid serial in that case, store the
serial of the latest touchpoint more persistently in GdkWaylandSeat itself,
so that we can still access it when the touchpoint has already ended.
This is a bit spaghetti right now, since seats and devices were
heavily entangled there are a number of crossed private API calls that
should ideally not be there.
Let this be a first step, so more bits may move from the seat
implementation to devices.
