Add support for EHB mode

[KaiN]

Lost Vikings 2 remake will be using EHB. so I need to implement it asap. The biggest problem is calculating proper copper move positions - not only one instruction more, but also Copper slows down in 6bpp.

• Preliminary support for 6BPP in SimpleBuffer and TileBuffer
• Test thoroughly 6BPP in SimpleBuffer: should work in scroll and noscroll mode
• Test thoroughly 6BPP in TileBuffer