support xcf >= 4 and xcf >= 11

[mothsART]

After using hachoir on modern xcf files, I found two issues :

1. xcf => 4 used a header property, named "precision" who is document here https://testing.developer.gimp.org/core/standards/xcf/#header
2. xcf >= 11 : offsets pointer (on layers, channels etc.) are write on 64 bits (instead of 32 before).

I've take care to keep compatibility on xcf older than version 4.

I don't claim to cover all cases but it's a good start.

You can fin my samples here : https://github.com/mothsART/xcf-rs/tree/master/tests/samples

[vstinner]

Would it be possible to add a very small sample file and some tests? See tests/test_parser.py.

[mothsART]

I've added 4 files : one for every major change. (and tests for each one).

[vstinner]

Merged, thank you.