There should be a way of turning on the drawing of zero width characters.
By default they may be drawn as the string composed of their Unicode name
e.g. "<RLM>", but the actual markup for each character should be resource
settable. In all calculations of characters widths etc this markup should
then be treated by pango as one cluster.

Once this option has been added to pango, GtkTextView should add an option
in its right click menu that allows turning on and off this option, in
order to easily visualizing the zero width characters.