Set amount by which WINDOW should be scrolled vertically to VSCROLL.
WINDOW nil means use the selected window. Normally, VSCROLL is a
non-negative multiple of the canonical character height of WINDOW;
optional third arg PIXELS-P non-nil means that VSCROLL is in pixels.
If PIXELS-P is nil, VSCROLL may have to be rounded so that it
corresponds to an integral number of pixels. The return value is the
result of this rounding.
If PIXELS-P is non-nil, the return value is VSCROLL.
(fn WINDOW VSCROLL &optional PIXELS-P)