Fix part of bug #13011 with display of lines that have a box face.

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.