bug#12401: scroll-preserve-screen-position broken with font height faces

[martin rudalics]

> Because all scroll-preserve-screen-position does is put point on
> certain window-relative pixel coordinates.  For this to follow a
> repeatable series of point values, you need the series of window-start
> positions to be repeatable, too.
>
> IOW, if you start with point P1 and window-start W1, and do a C-v
> followed by M-v, then setting point at the same pixel distance from
> the left-top window edge will get you back to P1 iff the window-start
> gets back at W1.

Yes.  Given the constraints of `scroll-preserve-screen-position' I
understand you now.

martin