.. 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.

Try harder to skip steps (reopen filter, reopen output) if the
AudioOutput is already open.

Un-protect the SharedPipeConsumer instance, and make it available in
the OutputThread only.  This gives more well-defined mutex protection.

Add another condition to Reopen() which allows keeping it open.

Make in_audio_format accessible only from within the OutputThread, and
add a new one for inter-thread communication.

Move the error logging to AudioOutput::PlayChunk().

Eliminate a dependency from MultipleOutputs on PlayerControl.

Reduce latency by stopping all AudioOutputs asynchronously.

Reduce shutdown latency by two commands per output.