I _think_ I've just fixed `font-overflow'. term->disptext now tracks

I _think_ I've just fixed `font-overflow'. term->disptext now tracks
the start of every contiguous run passed to do_text() or
do_cursor(), and arranges never to overwrite only part of such a run
on the next update.

I'm a bit worried about this checkin because I've also completely
revamped cursor handling: the cursor was previously being drawn
_outside_ the main loop over the display line, and is now drawn as
part of that loop when it gets to the cursor location. It _seems_ to
still work sensibly, even in complex cases involving LATTR_WIDE and
double-width CJK characters etc, but I won't be entirely happy until
it's had some beta use.

git-svn-id: http://svn.tartarus.org/sgt/putty@5003 cda61777-01e9-0310-a592-d414129be87e