Move the iterator a given number of characters visually, treating it as
the strong cursor position. If count is positive, then the new strong
cursor position will be count positions to the right of the old cursor
position. If count is negative then the new strong cursor position will
be count positions to the left of the old cursor position.
In the presence of bidirection text, the correspondence between logical
and visual order will depend on the direction of the current run, and
there may be jumps when the cursor is moved off the end of a run.