Bring in some of my scroll-optimisation stuff from the old Mac port.

Bring in some of my scroll-optimisation stuff from the old Mac port.
This introduces a new front-end function, do_scroll(), which is expected to
scroll a part of the physical display and cause repaint events for any
areas that couldn't be scrolled (e.g. because they were hidden).
scroll_display() is a wrapper around this which also updates disptext to
match.

Currently, scroll_display is only used in response to user scrollback requests
(via term_scroll()), but extending scroll() to use it as well should be
easy.

All of this is conditional on the front end's defining OPTIMISE_SCROLL, since
only the Mac front end currently implements do_scroll().

git-svn-id: svn://svn.tartarus.org/sgt/putty@2242 cda61777-01e9-0310-a592-d414129be87e