* Add craft-xcf: GIMP XCF reader from PhotoCraft
The format half of PhotoCraft's XCF import (crates/io/src/xcf.rs at abeb16c96): the header,
precisions, property lists with their length repairs, parasites, layer structures and item
paths, channels, and the uncompressed, RLE and zlib tiles. The mapping onto a document (blend
modes, surfaces, profiles, warnings) stays in the app.
* craft-xcf: an RLE run of zero length is an error
A long run (op 127 or 128) with a 16-bit length of 0 consumed input without producing output,
so a tile of them was work the pixel budget never charged.
Ported from storytold/photocraft#11138.
* craft-xcf: channel names are charged to the metadata budget
Layer names already were. Many channel pointers to one channel with a large name made a copy
per read, multiplying a small file into gigabytes of retained names.
Ported from storytold/photocraft#11138.
