Allow precision-scrolling nonselected windows when the minibuffer is resized

Allow precision-scrolling nonselected windows when the minibuffer is resized

* doc/lispref/windows.texi (Vertical Scrolling): Document new
`preserve-vscroll-p' parameter of `set-window-vscroll'.
* etc/NEWS: Announce new parameter.

* lisp/pixel-scroll.el (pixel-scroll-precision-scroll-down-page)
(pixel-scroll-precision-scroll-up-page): Use that parameter when
setting the vscroll.

* src/window.c (window_scroll_pixel_based, Fset_window_vscroll):
Adjust for new parameter.
* src/window.h (struct window): New flag `preserve_vscroll_p'.

* src/xdisp.c (redisplay_window): Preserve the vscroll inside
force_start on frozen windows with that flag set.  (bug#55312)