[integer] the byte index of a grapheme within the layout.
trailing
[logical] 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.
Value
A list containing the following elements:
line
[integer] location to store resulting line index. (which will
between 0 and pango_layout_get_line_count(layout) - 1)
x.pos
[integer] location to store resulting position within line
(PANGO_SCALE units per device unit)