window-scroll-functions are run before the window actually scrolls. So
just set a flag that it did so which is then picked up and cleared by
beacon--post-command. Then the redisplay isn't needed there, although
seldomly it seems post-command-hook is run before
window-scroll-functions. However, the redisplay in beacon--post-command
caused extreme slowdowns when repeatedly deleting text by pressing and
holding DEL in AUCTeX because (I think) every deleted char caused
fontification.
Also fix a problem where the blink occured in the current window whereas
some other window was scrolled, e.g., with the mouse or
scroll-other-window.