Move point to start of the screen line LINES lines down.
If LINES is negative, this means moving up.
This function is an ordinary cursor motion function
which calculates the new position based on how text would be displayed.
The new position may be the start of a line,
or just the start of a continuation line.
The function returns number of screen lines moved over;
that usually equals LINES, but may be closer to zero
if beginning or end of buffer was reached.
The optional second argument WINDOW specifies the window to use for
parameters such as width, horizontal scrolling, and so on.
The default is to use the selected window's parameters.
LINES can optionally take the form (COLS . LINES), in which case
the motion will not stop at the start of a screen line but on
its column COLS (if such exists on that line, that is).
`vertical-motion' always uses the current buffer,
regardless of which buffer is displayed in WINDOW.
This is consistent with other cursor motion functions
and makes it possible to use `vertical-motion' in any buffer,
whether or not it is currently displayed in some window.
(fn LINES &optional WINDOW)