[Cin] Aom AV1

[Andrea paz]

> In the changelog of ffmpeg 4.2 it was announced that Dav1d will be integrated. As far as I know it is faster than AOM.
>
I agree that Dav1d is a good thing to implement but, as has been said
in other messages, it's just a decoder. So aom will continue to be
necessary for the encoding. I propose to implement it with active
multithreading and not as it is by default, i.e. single thread. Unless
the reason they're back in the single thread is really important (I
don't know it).