Use of deprecated function in glg_cairo.c

[jonnyns]

The function gdk_display_get_device_manager() has been deprecated since version 3.20.
In glg_cairo.c lines 640-641
priv->device_manager = gdk_display_get_device_manager ( gtk_widget_get_display (widget) ); priv->device_pointer = gdk_device_manager_get_client_pointer (priv->device_manager);
should be replaced by
GdkSeat *seat = gdk_display_get_default_seat ( gtk_widget_get_display (widget) ); priv->device_pointer = gdk_seat_get_pointer(seat);
in order to compile correctly.