Way too many changes to document. Key things: history.tex reflects mo…

…st if not all changes from R7. The specification of the requirements for image_surface_data_type operations and output_surface_data_type operations is complete. experimental-io2d.tex reflects the API I think. Fixed some issues in graph-surf-surfstate.tex. Plenty more still to do but we've made good progress.

Commiting P0267R5.

Committing P0267R4.

P0267R3 revision accepted for pre-Kona mailing. Fixed up various erro…

…rs, added nullvalue_t type and replaced nullopt_t with it. Made surface virtual functions (except dtor) non-virtual per LEWG Oulu feedback. Renamed surface::save and surface::restore to surface::push_state and surface::pop_state, respectively. Changed display_surface::exit_show to take a std::chrono::duration instead of an int and made it compliant with the exception/noexceptions design. Also added an overload for that which takes no parameters. Switched the dashes typedef to a using dashes = ... version.

P0267R0 commit. Too many changes to summarize.