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().

[originally from svn r2242]

mac/macstuff.h

@@ -23,6 +23,8 @@ typedef void *Context; /* FIXME */
 #define WCHAR wchar_t
 #define BYTE unsigned char
 
+#define OPTIMISE_SCROLL
+
 /* To make it compile */
 
 #include <stdarg.h>