Return position at which display currently ends in WINDOW.
WINDOW must be a live window and defaults to the selected one.
This is updated by redisplay, when it runs to completion.
Simply changing the buffer text or setting `window-start'
does not update this value.
Return nil if there is no recorded value. (This can happen if the
last redisplay of WINDOW was preempted, and did not finish.)
If UPDATE is non-nil, compute the up-to-date position
if it isn't already recorded.
(fn &optional WINDOW UPDATE)