Add a dlo-based layout callback for use with lhp.

If lhp_dl_render() is used as the Lws Html/CSS parser callback, it
produces a display_list representation of the Html as DLOs, referencing the
appropriate CSS to drive the layout as best it can.

The display list can be rasterized on to an lws_display using the display's
state object

  lds->disp->blit(lds, (uint8_t *)&dl, &box);