(Fpos_visible_in_window_p): Adapt to new pos_visible_p.

(Fpos_visible_in_window_p): Adapt to new pos_visible_p.
Return row height and row number for partially visible rows.
Modify return value to a 2 element list for fully visible rows
and 6 for partially visible row.
(window_scroll_pixel_based): Use pos_visible_p directly instead of
Fpos_visible_in_window_p.  Fix auto vscrolling for partially visible lines.
Only vscroll backwards if already vscrolled forwards.  Unconditionally
vscroll forwards if PT is first (and only) line.  Set new window start
instead of scrolling at start/end of tall line.