Fix part of bug #13011 with display of lines that have a box face.
src/xdisp.c (handle_face_prop): Fix logic of computing
it->start_of_box_run_p.
(append_space_for_newline): If the glyph row is R2L, reset the
iterator's end_of_box_run_p flag before prepending the space glyph.
(extend_face_to_end_of_line): If the glyph row is R2L, reset the
iterator's start_of_box_run_p flag before prepending the stretch.
(append_glyph, produce_image_glyph, append_composite_glyph)
(append_stretch_glyph, append_glyphless_glyph): Reverse the
left_box_line_p and right_box_line_p flags of the glyph for R2L
glyph rows.