Add Panning Extension for Stereo formats

[ericoporto]

Attempt to fix #12

This is probably wrong somewhere and incorrect. Probably the most useful thing is the example.

I copied the testposition example to create a testpanning example with a red rectangle, moving the red rectangle to right or left modifies the panning to either right or left.

Here's a cool song that is stereo: https://opengameart.org/content/love-song-instrumental

[ivan-mogilko]

Note that this solution does not do panning (positioning) in the sense of OpenAL, what it does is channel balance. I think this would be a completely separate effect and property.

I have been in doubts myself for a while, but the difference is also explained in this comment in #12:
#12 (comment)