Merge duplicate code.

Prepare for adding state.

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

This prevented using the "volume_normalization" feature with some
codecs (e.g. mp3), because the normalization code requires 16 bit
samples.  If the codec happens to deliver formats other than S16, the
AutoConvert filter succeeds to initialize the conversion filter, but
the returned input audio format was wrong.

Worst-case allocation using MAX_CHANNELS.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

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.