Converts from an index within a `PangoLayout` to the onscreen position corresponding to the grapheme at that index.
The return value 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