(window_scroll_pixel_based): Move outside a

(window_scroll_pixel_based): Move outside a
multi-glyph character before setting new window start.

(window_scroll_pixel_based): Fix check for reaching BEGV.
Don't try to make last line fully visible if it is past end of window.

(window_scroll_pixel_based): Don't call Fbolp;
instead, see if the new start pos is at beginning of line.

(window_scroll_pixel_based): Exit the move_it_by_lines
loop whenever it stops making progress.