[Bug: 21.4.19] smooth vscroll-{up,down}

[Stephen J. Turnbull]

Jérôme Bouat writes:

 > But emacs can move smoothly the window,
 > it is simply not mapped in 'next-line' function.
 > For the expected result, make emacs evaluate:
 > (dotimes (i 10) (set-window-vscroll nil (* i 0.1)) (sit-for 0.05))

`set-window-vscroll' is an Emacs function not present in XEmacs.  Nor
is there equivalent functionality as far as I know.