Converts from an index to the onscreen position corresponding to the
grapheme at that index, which is represented as a rectangle. Note that
x is always the leading edge of the grapheme and x+width the trailing
edge of the grapheme. If the direction of the grapheme is right-to-left,
then width will be negative.
Returns:
([ "x": x coordinate, "y": y coordinate,
"width": width of the rectangle, "height": height of the rectangle ])