Converts an index within a line to a X position.
| this |
a `PangoLayoutLine` |
| index_ |
byte offset of a grapheme within the layout |
| trailing |
an integer indicating the edge of the grapheme to retrieve the position of. If > 0, the trailing edge of the grapheme, if 0, the leading of the grapheme |
| x_pos |
location to store the x_offset (in Pango units) |