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.
Rename Bitmap::realwidth to pitch.
Use the SDL logging features instead of our own.
Remove unused Video_bitmap_X11::get_bitmap method.
Initial cut at an SDL conversion.