For a `struct window', changing the type of `last_point', `last_modified'
and `last_overlay_modified' from Lisp_Object to appropriate integral
value eliminates a lot of useless Lisp_Object <-> {EMACS_INT, ptrdiff_t}
conversions in a window and redisplay code; and, of course, lesser Lisp_Objects
means a bit faster GC. OK to install?

Dmitry