Gets the TextIter at the start of the line containing the coordinate y.
y is in buffer coordinates, convert from window coordinates with
window_to_buffer_coords. If non-null
, line_top will be filled with the coordinate of the top edge of the line.
| this |
a TextView |
| target_iter |
a TextIter |
| y |
a y coordinate |
| line_top |
return location for top coordinate of the line |