avviddec: use a more conservative default number of threads

When doing per-frame multithreading, FFmpeg introduces a delay of one frame per thread. On a machine with a high number of cores, the default maximum of 16 can easily lead to deadlocks in multiqueue.

The ideal situation would have multiqueue query a latency downstream to add to its maximum, but that's not easily implemented.

For now simply use a lower default maximum of 4, this should usually still be enough for real-time playback purposes, and can of course be set by the user through the max-threads property.