Resynchronization of Audio/Video on audio filter changed

[bylee20]

As I said in #1678, I'm tryng to prefetch audio data.
I found that I got the A-V desync when a filter which has delay is inserted.
This is also true for scaletempo which is built-in filter of mpv.

For instance, let's say a filter whose delay is 0.1sec is inserted. Then, I get 0.1 seconds of AV desync immediately and it decrease gradually. Usually, delay which is introduced by filter is less than 1 second and the out-of-sync state goes away quickly.

However, in my case, I need a huge delay over than 10 seconds and it takes several seconds to reduce that desync. Futhermore, this delay is accumulated whenever toggling the filter.

So, I think it would be good to resynchronize A-V when audio delay is introduced by filter.
Or, maybe it's more useful to introduce a command to resynchronize A-V at anytime for general purpose. For now, I'm enforcing to synchronize A-V by seeking.

[ghost]

This shouldn't happen if the filter adjusts af->delay dynamically, but even then removing the filter will mess up A/V sync.

I'm not sure about the solution yet.