(init_iterator): Initialize it->start to position

(init_iterator): Initialize it->start to position
before reseating (in case start position is invisible).
(init_to_row_start): Set it->start to row-start.
(redisplay_window): Accept optional_new_start if start position
is invisible (in which case IT_CHARPOS overshoots PT).
(display_line): Setup row->start from it->start (rather than
it->current which is wrong if first char on line is invisible).
When done, reseat it->start to it->current (= start of next row).
(expose_area): Fix exposure of text area when first char (e.g. TAB)
is only partially visible.