demux: asynchronous seeking

This tells the demuxer thread that it should seek, instead of waiting
until the demuxer thread is ready.

Care has to be taken about the state between seek request and actual
seeking: newly demuxed packets have to be discarded. We can't just
flush when doing the actual seek, because the user thread could read
these packets.

I'm wondering if this could lead to issues due to relaxed ordering of
operations. But it should be fine, since seeking influences packet
reading only, and seeking is always strictly done before that.

Currently, this will have no advantages; unless audio is disabled. Then
seeking as well as normal playback can be non-blocking.

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