Continue work on R2L paragraphs in GUI sessions.

Continue work on R2L paragraphs in GUI sessions.

 xdisp.c (set_cursor_from_row): Don't be fooled by truncated
 rows: don't treat them as having zero-width characters.  Improve
 comments.
 (extend_face_to_end_of_line): Use window_box_width to compute the
 width of the stretch glyph, to account for margins.  If this is
 the last glyph row, use DEFAULT_FACE_ID, to avoid painting the
 rest of the window with the region face.