A qsort-style function that returns negative if this is less than rhs,
positive if this is greater than rhs, and 0 if they’re equal.
Ordering is in character offset order, i.e. the first character in the buffer is less than the second character in the buffer.
| this |
a `GtkTextIter` |
| rhs |
another `GtkTextIter` |
|
-1 if this is less than |