Bug fixes:
- I was decoding the quadrature channel of NTSC colour as though it had PAL-style alternated phase. Which for various reasons caused half of the Apple II high-resolution colours to be swapped. This is now fixed;
- the validation applied to ZX81 .p files was too strict, causing at least one title (Humpty Dumpty) to be rejected. It has now been loosened;
- my attempt at 1bpp texture decoding on the GPU had precision issues in practice, affecting the output of the Apple II and the ZX80/81. At least for now I've switched to 1 byte per pixel;
- a port decoding error put the ZON-X AY read register in the wrong location in the ZX81 IO map. It's now where it should be; and
- it turned out that the machine-visible copy of AY registers (as audio is generated on a separate thread, there are two copies) wasn't being initialised correctly. It is now.