Proxy generation modifies the project, causing graph snapshots to be
rewritten. The old implementation deleted the previous snapshot
immediately, even though queued render jobs still held its path. The
worker then failed to load the deleted file.
Add per-path reference counting:
- Increment when a job is created with a snapshot path.
- Decrement when the job is processed, cancelled or removed.
- Only delete a snapshot when its reference count reaches zero and it
is no longer the cached snapshot for the project.
Also keep snapshots in the system temp directory as before, since the
location itself was not the bug.
Mike-Solar