* xdisp.c (start_display): Don't reseat to next visible line start

* xdisp.c (start_display): Don't reseat to next visible line start
if current start position is in a string or image.
(move_it_vertically_backward): Be sure to move out of strings and
images when moving it2 forward.
(move_it_by_lines): When moving forward, move to next buffer
position if we end up in a string or image.  When moving backward,
count rows moved over when moving to start of current row in case
row starts in middle of a string or image.  Also move further
backward if we end up in a string or image.
(try_cursor_movement): if overlay string spans multiple lines,
move backward to set cursor on start of an overlay string.
(cursor_row_p): Row is ok if cursor is at newline from string, but
string starts on this line (so we always position cursor at start
of string).