Enchancement: Olivetti M24-like modes

[skiselev]

Olivetti M24 had mostly CGA compatible display adapter with several enhancements:

• The display resolution was 640x400
• Standard CGA graphics modes used scan doubling, very similar to Graphics Gremlin
• Standard CGA text modes used 8x16 font, similar to that of VGA adapters
• It supported an additional 640x400 monochrome graphics mode

It would be nice to implement at least the 640x400 / 8x16 font text mode. It is much more readable than the default CGA 8x8 font Graphics 640x400 mode is nice too, but I don't think it was widely used