Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Return the pointer given by PcmBuffer::Get() instead of reaching into
the PcmBuffer object.

Fixes a clang warning.

Make the audio_format argument non-const.  Allow the open() method to
modify it, to indicate that it wants a different input audio format
than the one specified.  Check that condition in chain_filter_open(),
and fail.

Detect misconfiguration during MPD startup, not when playback begins.

Indicate success and error.