- Tracer now iterates over blocks in disk order.
- Packer copies files per directory, in a separate thread.
- Packer only copies files if they don't exist yet.
- Packer also copies file permissions.
This simplifies blend file handling, ensuring that blend files are only
opened once. Otherwise it would be harder to handle things like dependency
diamonds (libraries that are referenced via multiple paths through multiple
other libraries).
The expansion process follows pointers and library links to construct
the full set of actually-used data blocks. This set consists of all data
blocks in the initial blend file, and all *actually linked-to* data
blocks in linked blend files.
I've also removed non-recursive dependency listing.
