Retrieves a particular line from a gtk.Layout
This is a faster alternative to Layout.getLine,
but the user is not expected
to modify the contents of the line (glyphs, glyph widths, etc.).
the index of a line, which must be between 0 and
<literal>pango_layout_get_line_count(layout) - 1</literal>, inclusive.
PangoLayoutLine, or NULL if the index is out of
range. This layout line can be ref'ed and retained,
but will become invalid if changes are made to the
gtk.Layout No changes should be made to the line.