mpg123-decoder: mono streams should return one channel

[hackerbreeze]

why the mp3 decoder yields the wrong number of channels when decoding single-channeled mp3 files?
the right channelData.length should be 1 but it says 2.

[eshaz]

Currently, it decodes mono MP3 files into stereo. The two channels are identical though, you can just use the left channel for mono MP3.

That being said, it makes sense to only return one channel though if the MP3 is mono, especially since the other decoders work this way. I'll work on an update for this.