Support for speaker-based surround systems

[Hiradur]

It would be nice if speaker-based surround systems were supported. I suggest that if the browser reports more than 2 output channels, an Ambisonics decoder is chosen based on the number of channels, e.g.:

4 channels -> square decoder
6 channels (5.1) -> decoder for ITU 5.1 speaker layout
8 channels (7.1) -> regular decoder for 6-speaker hexagon layout

For 7.1, a user-selectable decoder for the 3D7.1 speaker layout could also be provided perhaps.