We were checking the columns of the whole selection instead of the
the specfic line were modifying. Because of this, the selection
remained if the selection's column on another line was less than
the cursor.
This optimization was broken since who-knows-when. Now we once again do
our best to only repaint the lines that had the "dirty" flag set.
This dramatically reduces the amount of work done by an idle Terminal
since the cursor blinking won't redraw the whole window anymore. :^)
