This function is given a barely setup GdkEvent, so the GdkDevice field
is still unset, causing warnings and misbehaviors when the position
is queried for it.
Given that the wintab GTK+ code seems to rely somewhat hard on the wintab
device managing the pointer cursor, query the pointer position from the
pointer itself.
https://bugzilla.gnome.org/show_bug.cgi?id=743330