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.
Remove the unneeded Video::flip method.
Remove unneeded Video::start_frame method.
Inline the calls to {start,end}_frame.
Remove the unused start_game function from wadder.
Remove unused alt_tab global variable.
Fix the type of Video::newpal.