Fix bug #9470 with slow redisplay in huge single-paragraph buffers.

Fix bug #9470 with slow redisplay in huge single-paragraph buffers.

 src/bidi.c (MAX_PARAGRAPH_SEARCH): New macro.
 (bidi_find_paragraph_start): Search back for paragraph beginning
 at most MAX_PARAGRAPH_SEARCH lines; if not found, return BEGV_BYTE.
 (bidi_move_to_visually_next): Only trigger paragraph-related
 computations when the last character is a newline or at EOB, not
 just any NEUTRAL_B.
 src/xdisp.c (reseat_at_next_visible_line_start): Keep information
 about the current paragraph and restore it after the call to
 reseat.