Fix fallout from "Qnil is zero" change in the display engine. (Bug#19535)

Fix fallout from "Qnil is zero" change in the display engine.  (Bug#19535)

 src/xdisp.c (next_element_from_c_string): Use Lisp integer zero as
 the object.
 (set_cursor_from_row, try_cursor_movement, dump_glyph)
 (insert_left_trunc_glyphs, append_space_for_newline)
 (extend_face_to_end_of_line, highlight_trailing_whitespace)
 (find_row_edges, ROW_GLYPH_NEWLINE_P, Fmove_point_visually)
 (Fbidi_resolved_levels, produce_special_glyphs)
 (rows_from_pos_range, mouse_face_from_buffer_pos)
 (note_mouse_highlight): Use nil as the object for glyphs inserted
 by the display engine, and test with NILP instead of INTEGERP.
 src/w32fns.c (Fx_show_tip): Use NILP to test for glyphs inserted by
 the display engine.
 src/xfns.c (Fx_show_tip): Use NILP to test for glyphs inserted by
 the display engine.
 src/dispextern.h (struct glyph, struct it): Update comments for the
 OBJECT members.