Initial support for bidirectional editing.

Initial support for bidirectional editing.

 Makefile.in (obj): Include bidi.o.
 (bidi.o): New target.
 makefile.w32-in (OBJ1): Add $(BLD)/bidi.$(O).
 ($(BLD)/bidi.$(O)): New target.
 bidi.c: New file.
 buffer.h (struct buffer): New members bidi_display_reordering
 and bidi_paragraph_direction.
 buffer.c (init_buffer_once): Initialize bidi_display_reordering
 and bidi_paragraph_direction.
 (syms_of_buffer): Declare Lisp variables bidi-display-reordering
 and bidi-paragraph-direction.
 (Fbuffer_swap_text): Swap the values of
 bidi_display_reordering and bidi_paragraph_direction.
 dispextern.h (BIDI_MAXLEVEL, BIDI_AT_BASE_LEVEL): New macros.
 (bidi_type_t, bidi_dir_t): New types.
 (bidi_saved_info, bidi_stack, bidi_it): New structures.
 (struct it): New members bidi_p, bidi_it, paragraph_embedding,
 prev_stop, base_level_stop, and eol_pos.
 (bidi_init_it, bidi_get_next_char_visually): New prototypes.
 (IT_STACK_SIZE): Enlarge to 5.
 (struct glyph_row): New member reversed_p.
 <string_buffer_position>: Update prototype.
 (PRODUCE_GLYPHS): Set the reversed_p flag in the iterator's
 glyph_row if bidi_it.paragraph_dir == R2L.
 (struct glyph): New members resolved_level and bidi_type.
 dispnew.c (direct_output_forward_char): Give up if we need bidi
 processing or buffer's direction is right-to-left.
 (prepare_desired_row): Preserve the reversed_p flag.
 (row_equal_p): Compare the reversed_p attributes as well.
 xdisp.c (init_iterator): Initialize it->bidi_p.  Call
 bidi_init_it and set it->paragraph_embedding from the current
 buffer's value of bidi_paragraph_direction.
 (reseat_1): Initialize bidi_it.first_elt.
 (set_iterator_to_next, next_element_from_buffer): Use the value of
 paragraph_embedding to determine the paragraph direction.
 (set_iterator_to_next): Under bidi reordering, call
 bidi_get_next_char_visually.  Call bidi_paragraph_init if the
 new_paragraph flag is set in the bidi iterator.
 (next_element_from_buffer): If bidi_it.first_elt is set,
 initialize paragraph direction and find the first character to
 display in the visual order.  If reseated to a middle of a line,
 prime the bidi iterator starting at the line's beginning.  Handle
 the situation where we overstepped stop_charpos due to
 non-linearity of the bidi iteration.  Likewise for when we back up
 beyond the previous stop_charpos.  When moving across stop_charpos,
 record it in prev_stop.
 (display_line): Set row->end and it->start for the next row to the
 next character in logical order.  Always extend reversed_p rows to
 the end of line, even if they end at ZV.  Copy the reversed_p flag
 to the next glyph row.  Keep calling set_cursor_from_row for
 bidi-reordered rows even if we already have a possible candidate
 for cursor position.  Set row_end after all the row's glyphs have
 been produced, by looping over the glyphs.  Record the position
 after EOL in it->eol_pos, and use it to set end_pos of the last
 row produced for a continued line.
 <Qright_to_left, Qleft_to_right>: New variables.
 (syms_of_xdisp): Initialize and staticpro them.
 (string_buffer_position_lim): New function.
 (string_buffer_position): Most of code moved to
 string_buffer_position_lim.  Last argument and return value are
 now EMACS_INT; all callers changed.
 (set_cursor_from_row): Rewritten to support bidirectional text and
 reversed glyph rows.
 (text_outside_line_unchanged_p, try_window_id): Disable
 optimizations if we are reordering bidirectional text and the
 paragraph direction can be affected by the change.
 (append_glyph, append_composite_glyph)
 (produce_image_glyph, append_stretch_glyph): Set the
 resolved_level and bidi_type members of each glyph.
 (append_glyph): If the glyph row is reversed, prepend the glyph
 rather than appending it.
 (handle_stop_backwards): New function.
 (reseat_1, pop_it, push_it): Set prev_stop and base_level_stop.
 (reseat): call handle_stop_backwards to recompute prev_stop and
 base_level_stop for the new position.
 (handle_invisible_prop): Under bidi iteration, skip invisible text
 using bidi_get_next_char_visually.  If we are `reseat'ed, init the
 paragraph direction.  Update IT->prev_stop after skipping
 invisible text.
 (move_it_in_display_line_to): New variables prev_method
 and prev_pos.  Compare for strict equality in
 BUFFER_POS_REACHED_P.
 (try_cursor_movement): Examine all the candidate rows that occlude
 point, to return the best match.  If rows are bidi-reordered
 and point moved backwards, back up to the row that is not a
 continuation line, and start looking for a suitable row from
 there.
 term.c (append_glyph): Reverse glyphs by pre-pending them,
 rather than appending, if the glyph_row's reversed_p flag is set.
 Set the resolved_level and bidi_type members of each glyph.
 .gdbinit (pbiditype): New command.
 (pgx): Use it to display bidi level and type of the glyph.
 (pitx): Display some bidi information about the iterator.
 (prowlims, pmtxrows): New commands.
 files.el: Make bidi-display-reordering safe variable for boolean
 values.
 mule.texi (International): Mention support of bidirectional editing.
 (Bidirectional Editing): New section.
 HELLO: Reorder Arabic and Hebrew into logical order, and
 insert RLM before the opening paren, to make the display more
 reasonable.  Add setting for bidi-display-reordering in the local
 variables section.
 NEWS: Mention initial support for bidirectional editing.