Moves this iterator forward by a single cursor position. Cursor
positions are (unsurprisingly) positions where the cursor can appear.
Perhaps surprisingly, there may not be a cursor position between all
characters. The most common example for European languages would be
a carriage return/newline sequence. For some Unicode characters, the
equivalent of say the letter "a" with an accent mark will be
represented as two characters, first the letter then a "combining mark"
that causes the accent to be rendered; so the cursor can't go between
those two characters. See also Pango.LogAttr and pango_break().