(follow-calc-win-end): Use with-selected-window.

(follow-calc-win-end): Use with-selected-window.
(follow-windows-start-end, follow-pos-visible)
(follow-windows-aligned-p): Code cleanup.
(follow-select-if-visible): Try to avoid partially-visible lines.
(follow-select-if-visible-from-first): Call follow-redisplay and
move point to destination.
(follow-redisplay): New arg, to keep selected window unchanged.
(follow-post-command-hook): In final check for destination
visibility, use window-start and window-end instead of the less
accurate follow-pos-visible.  If the selected window is redrawn,
tell follow-redisplay to preserve it.