Remove the unused Stringtable::operator=.
Remove the DirectX backend and make X11 the default.

The X11 backend will be evolved into an SDL backend.
Remove the self-drawn cursor.
Remove support for arbitrary color depths.

This never worked, so no great loss.
Remove the unneeded Video::clean_up method.
Remove unused Video_bitmap_X11::get_bitmap method.
Switch error.cpp to use SDL_Log.
Constify a few member variables.
Pull Video_bitmap::box up.
Make delete_obj private.
Remove the pointless quit_game function.
Replace getmsec with SDL_GetTicks.
Provide an indirection for creating Video_bitmap.
Mark Palette::set as const.
Simplify the input classes a bit.
Make more members of Video const.
Minimal mostly video port to SDL.

Many asserts left, no keyboard input, but it seems good enough to replay games.
Fix memory ownership of Video::vb.
Remove unused Video_bitmap::setmem method.
Removed the unused Video::{lock,unlock} methods.