Fix display of R2L lines in partial-width windows.

Fix display of R2L lines in partial-width windows.

 src/xdisp.c (init_iterator): Don't use it->bidi_p before it is
 assigned the correct value.
 (extend_face_to_end_of_line): Account for truncation and
 continuation glyphs in R2L rows when one of the fringes is not
 displayed.
 (display_line): Don't assign negative X offset to a row if we are
 going to produce a truncation glyph for it.  When handling
 truncated R2L rows, consider the width of the left fringe instead
 of the right one.
 (produce_special_glyphs): Fix bogus assignments.