Retrieves a particular line from a gtk.Layout
Use the faster Layout.getLineReadonly if you do not plan
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