(compute_line_metrics): If first line's physical ascent

(compute_line_metrics): If first line's physical ascent
is larger than its logical ascent, use the physical ascent, and
make the row taller.  Set row's overlapping_p flag.
(init_iterator): Reset physical line height info
after producing special glyphs.
(display_toolbar_line): Set physical line height info.
(compute_line_metrics): Ditto.
(display_line): Ditto.
(display_string): Ditto.
(reseat_at_next_visible_line_start): New parameter
on_newline_p.
(set_iterator_to_next):  After delivering last char
from display vector, reseat on next visible line start if
dpvec_char_len < 0.
(next_element_from_buffer): Set dpvec_char_len to -1 for
selective display.