Support 5.1 Vorbis files

[duysqubix]

The code no longer assumes Vorbis files are stereo (they may have up to 255 channels), which caused e.g. mono files to play at twice their speed.

The current strategy for files with more than 2 channels is to only play the first and last channels, and drop the rest.

Original issue: faiface/beep#154

[MarkKremer]

The mono part is implemented in #10.

In the original PR I said that if we would like to support 3+ channels that we should pick the front left and front right channels instead of the first and last channels. However, that drops some channels still which I'm not sure if I'm a fan of. I propose we wait until 3+ channel support (#9) is implemented and then this can be implemented the proper way.

If someone needs this sooner, please comment. It's not a decision set in stone.

[K4thos]

If someone needs this sooner, please comment. It's not a decision set in stone.

We're considering switching from our beep fork to gopxl/beep, but not having support for 5.1 would be a regression compared to the current release of our engine. Please consider merging this PR as a temporary solution, until a better one is ready: faiface/beep#154

[MarkKremer]

Sure!

Does your fork happen to have implemented my suggestions (faiface/beep#154 (comment))?

[K4thos]

Thanks! I'm afraid not - original content of PR 154 matches what's in our fork.

[MarkKremer]

Does #163 work for you and your usecase @K4thos?

[K4thos]

yes, looks good, thanks!