I have been in contact with Ville Pulkki and it is clear that in 2D
having an angle of more than 170 degrees between two adjacent speakers
causes that pair to be ignored -- at 180 degrees an internal matrix
becomes singular and the algorithm breaks down.  This means that the
configuration of two subs at -90 and +90 will never work in vbap.

I have adjusted the code to make the maximum angle 179 degrees and
added a note in the manual

AFAIA vpab now works with multiple configurations.  I may add more
warnings about bad configurations but otherwise it is finished for
now.

==John ffitch