Speed up redisplay of long truncated lines

Speed up redisplay of long truncated lines

* src/xdisp.c (forward_to_next_line_start): Fix logic of
interpreting the result of Fnext_single_property_change.
(reseat_at_next_visible_line_start): When ON_NEWLINE_P is zero,
pass NULL to 'forward_to_next_line_start', to avoid costly bidi
iteration when none is needed.  This speeds up redisplay of very
long lines under 'truncate-lines'.