Move the code that adjusts the layout's x-offset for fixed-width text
boxes and RTL, centered- and right justified text. It was living in
gimp_text_layout_render() and was correctly applied at rendering time,
so everything looked fine. Move it to gimp_text_layout_position() so
it becomes part of the layout's permanent properties, is reported by
gimp_text_layout_get_offsets() and thus used by the text tool which
can draw cursor and selection at the right place.