Handle ColourDefault in extended output modes.

This also makes ColourBlack etc. correct in 256-colour mode.

I set ColourDefault to 0x100 in stead of 0,
and AttrBold, AttrUnderline etc have been shifted,
but this should be an internal change only.

I'm not sure if this is the best way to handle it, but with 256 colour mode there's no space in 0x00-0xff for the default colour specifier, so i just used a higher bit for it.