List of functions to call before redisplaying a window with scrolling.
Each function is called with two arguments, the window and its new
display-start position. Note that these functions are also called by
`set-window-buffer'. Also note that the value of `window-end' is not
valid when these functions are called.
Warning: Do not use this feature to alter the way the window
is scrolled. It is not designed for that, and such use probably won't
work.