Redisplay resets vscroll when window start changes

[Po Lu <luangruo@yahoo.com>]

I was implementing pixel-based scrolling based on XInput 2 pixel scroll
events earlier, and i noticed this line in xdisp.c:

 /* Handle case where place to start displaying has been specified,
    unless the specified location is outside the accessible range.  */
  if (w->force_start)
    {
      /* We set this later on if we have to adjust point.  */
      int new_vpos = -1;

      w->force_start = false;
-->   w->vscroll = 0;
      w->window_end_valid = false;

It resets the vscroll whenever window start changes, which is annoying
if you, for example, recenter the window during pixel scroll.

Is it OK to control whether or not the vscroll is reset there based on a
variable or a window parameter?  It would be very convenient to have
such a feature.

Thanks.