The basic idea is that such new images will be packed into the .blend file
when saving all modified images from the quit dialog. To make this workflow
nicer a number of changes were made to how this type of packed image is
handled.
* "Save Modified Images" now packs generated images into the .blend file.
* "Save As" for packed images now automatically unpacks the image, so that
it's easy to save automatically packed images. "Save Copy" keeps it packed.
* "Save" for packed images now re-saves the image into the .blend file, so
that it's effectively the equivalent of "Save" for non-packed images.
* Empty image filepaths are no longer remapped when saving the .blend file.
Previously it would become e.g. "//../../" which makes no sense for generated
images with no filepath yet.
* Hide unpack button and filepath for such packed images with no filepath.
Unpacking does not work in a predictable way without a filepath, better
to just "Save As".
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