Update rav1e to v0.6.1

[EwoutH]

rav1e released version v0.6.1 today! It offers over a year of improvements over the v0.5.0 releases, including a lot of new assembly code from the dav1d project.

For Windows, there are two release binaries available: rav1e.exe, which only requires SSE2 and thus supports almost all x86_64 processors, and rav1e-ch.exe, which was build using the x86-64-v3 target, and thus supports/requires AVX2.

It might be interesting for multi-threaded capabilities, to encode multiple tiles simultaneously. I would recommend adding --tiles 8 to the command line, to beter use the multi-threading capabilities of rav1e, by encoding eight tiles simultaneously.

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By the way, it's a good day for AV1, since SVT-AV1 has planned their v1.4.0 release for today!