But also don't wait for a buffer on both pads, which might take forever in case
of gaps in one of the streams.
The muxer can only advance the time if it has a timestamped buffer that can be
output, otherwise it will just busy-wait and use up a lot of CPU.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7871>
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