Fix bug #16151 with background of display margins.

Fix bug #16151 with background of display margins.

 src/xdisp.c (extend_face_to_end_of_line): Extend background of
 non-default face in margin areas as well.  (Bug#16151)
 (display_line): Call extend_face_to_end_of_line for continued
 lines as well, if the display margins have non-zero width.
 (set_glyph_string_background_width): When needed, set the
 extends_to_end_of_line_p flag on glyph strings to be drawn in
 margin areas, not only in the text area.