Converts from an index within a Layout to the onscreen position corresponding to the grapheme at that index, which is represented as rectangle.
Note that pos->x is always the leading edge of the grapheme and pos->x + pos->width the trailing edge of the grapheme. If the directionality of the grapheme is right-to-left, then pos->width will be negative.
byte index within this
rectangle in which to store the position of the grapheme