dav1d: limit the number of extra frames needed by the decoder

The i_extra_picture_buffers is used to add pictures to the pool that the core will allocate. dav1d is actually using n_threads frames. And the core is allocating 10 frames per default for AV1. So we need to add the missing ones.

This is done only for dav1d 1.0. Older dav1d may have needed more frames so we leave the code untouched.

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v3: update dav1d to make use of the new API to get this value properly, when it's available