Format and read/write GAP#3 values are different, but we only get one

[pelrun]

Looks like we get the format GAP#3 in the disk image. I'm setting both to this value, but the read/write value appears to normally be about half the format one. (42 for a format GAP#3 of 80/82).

That means there's probably a risk of overwriting the ID of a following sector. Easiest fix is just to set the default write GAP#3 value when we get an expected format one; the trick is figuring out what behaviour to use in the other cases. Pinball Dreams, which triggered this in the first place, seems to use 42 for the format gap!

Need to find some weird formats (Bigbonzo's worth a look) to test against.