Initial reimplementation of calculating line edge positions in bidi lines.

Initial reimplementation of calculating line edge positions in bidi lines.

 dispextern.h (struct glyph_row): New members minpos and maxpos.
 (MATRIX_ROW_START_CHARPOS, MATRIX_ROW_START_BYTEPOS)
 (MATRIX_ROW_END_CHARPOS, MATRIX_ROW_END_BYTEPOS): Reference minpos
 and maxpos members instead of start.pos and end.pos, respectively.
 xdisp.c (display_line): Compare IT_CHARPOS with the position in
 row->start.pos, rather than with MATRIX_ROW_START_CHARPOS.
 (cursor_row_p): Use row->end.pos rather than MATRIX_ROW_END_CHARPOS.
 (try_window_reusing_current_matrix, try_window_id): Use
 ROW->minpos rather than ROW->start.pos.
 (init_from_display_pos, init_iterator): Use EMACS_INT for
 character and byte positions.
 (find_row_edges): Renamed from find_row_end.  Accept additional
 arguments for minimum and maximum buffer positions seen by
 display_line for this row.  Don't use iterator to find the
 position following the maximum one; instead, increment the
 position found by display_line directly.
 (display_line): Record minimum and maximum buffer positions for
 glyphs in this row.  Record the position of the newline that
 terminates the line.
 dispnew.c (increment_row_positions, check_matrix_invariants):
 Increment and check row->start.pos and row->end.pos, in addition
 to MATRIX_ROW_START_CHARPOS and MATRIX_ROW_END_CHARPOS.