Gets the `GtkTextIter` at the start of the line containing the coordinate y.
y is in buffer coordinates, convert from window coordinates with [[email protected]_to_buffer_coords]. If non-
null, line_top will be filled with the coordinate of the top edge of the line.
| this |
a `GtkTextView` |
| target_iter |
a `GtkTextIter` |
| y |
a y coordinate |
| line_top |
return location for top coordinate of the line |