We're using std::chrono::steady_clock now.  No need to duplicate code.

.. and remove the obsolete method OpenFilter().

Convert assertion to runtime check; this assertion could fail when the
output was closed due to an error before CLOSE arrived.

The filter can take some time to finish, even more so on a weak
machine with a saturated CPU.  By not holding the mutex during that
time, we can reduce PlayerThread latency a lot, because that thread
needs to synchronize a lot with all outputs.

Don't expose MusicChunk instances, provide higher-level access to
chunk contents.