[vpdq] Ensure VpdqHasher joins threads on destruction

[ianwal]

Summary

VpdqHasher currently doesn't join its hashing threads on destruction.

This is a problem if VpdqHasher::finish() is not called and VpdqHasher is destroyed. The threads will be destroyed while not joined which causes an abort.

Also remove the template specialization code so that this file doesn't depend on FFmpeg libraries at all. These aren't necessary after #1605.

I spotted this while integrating vpdq in another project. I was using Python bindings and when an exception occurred from Python during hashing the entire program would deadlock.

Test Plan

Current regtest doesn't cover this because finish() is always called in filehasher.cpp. This would be something for a unit test.

But I tested by removing the finish() call in filehasher.cpp and ran regtest with TSan. Before there was an abort and now there isn't.

Also tested with my other project and it fixed the deadlock issue immediately.

Thanks!

[Dcallies]

Thanks again @ianwal for a clean writeup, including your test plans. They help a lot in making these easy to review, despite my lack of C++ knowledge!