elm_code_widget: when the position is updated, set new geom to cursor

in order to do that, we need to ensure that the internal box is
calculated, otherwise the position is still not updated correctly.
This fixes wrong cursor positions in edi.

fixes T8214

Reviewed-by: Stefan Schmidt <stefan@datenfreihafen.org>
Differential Revision: https://phab.enlightenment.org/D10225