Fixes:
- an error in disk write accumulation affecting the Electron and Oric; reformatting a disk and then writing to a sector would make later sectors inaccessible;
- PAL gamma is now enforced for PAL machines, which makes a visible difference to the Oric, Vic-20 and Atari 2600;
- a false assumption about counter reloading in the AY that could produce scratchy audio in the Oric; and
- the Vic's colour palette if configured as an NTSC machine.
Also introduces an early, provisional version of Amstrad CPC emulation. It's not yet cycle perfect, so further work is required.