The format half of PhotoCraft's OpenRaster import and export (crates/io/src/ora.rs at
abeb16c96): the archive and its mimetype, stack.xml as a tree of layers and stacks with the
attributes the file has, the 0.0.6 group isolation rule, and a writer. PNG coding, blend-mode
mapping, rendering and warnings stay in the app.
stack.xml is read with craft-xml and the archive with the zip crate.
