fix: improve the decoding

[hooke007]

We do not need much too many threads to decode when requesting.

[hooke007]

[christoph-heinrich]

It has less cpu usage, but it's also slower.

[hooke007]

Comparing with the current branch, I didn't notice the diff about the preview presenting.

[christoph-heinrich]

I'm surprised how little difference it makes most of the time, but with something like 4k h265 it becomes very noticeable for me.

multi.thread.mp4

single.thread.mp4

[hooke007]

How about two threads?

[christoph-heinrich]

threads.2.mp4

threads.3.mp4

[hooke007]

I added the other solution to reduce the CPU's pressure but faster seeking.

[natural-harmonia-gropius]

Just put default options.hwdec to "no", it's good to be able to modify this. e.g. low-end graphics cards unable to decode two 8k 10bit av1 streams at same time, driver will crash.

[hooke007]

Isn't it 'no' now?

[natural-harmonia-gropius]

mybad.