The project carried the layer stack but not the user's place in it: a reopened
project landed on slot 0 in Standard, whatever they had been editing. The active
layer and Standard/Pro now live on the volume, next to the rest of its options,
and come back with it. Docked/undocked goes to the application config instead -
it describes the user's window, not the model, so opening someone else's project
must not move their panel.
A layer whose image_data was empty at save time was written with no texture and
came back with none. It now falls back to the file the layer came from, which the
importer put in the user's own texture library, so a project reopened on the same
machine keeps its pixels.
The texture is always stored as .png: the importer converts whatever the user
picked, so the extension was being taken from a name that no longer described the
bytes.
Also adds v2_flip_edges to the cereal field list, which the undo stack was
silently dropping.