VBAP backend re-work (part one):

* fix azimuth, don't clamp but map to [0,1]
* prepare elevation (10+ speakers)
VBAP backend re-work (part two): speaker positioning

* clean up source (whitespace)
* fix speaker 3x3 matrix iteration
* update math to go along with Ardour Cartesian -- fixes rounding errors
* fix division by zero in cross_prod()
* disable old debug output

(NB PBD::spherical_to_cartesian() returns
  3.7494e-33, 6.12323e-17, 1 for azimuth 90 elevation 90 distance 1
while it should return
  0.000000, 0.000000, 1 for azimuth 90 elevation 90 distance 1
IOW  cos(90.0 * 2.0 * M_PI / 360.0) != 0
Cause unknown. This is currently worked around check in vec_length()
)
VBAP GUI depends on signal-position (not parameter changes)
update default speaker position list
VBAP rework (part III): fix position computation backend & GUI
add spinbox to control width in VBAP GUI,..
redefine Pi :)
hide speaker-config in menu (it's not implemented yet)
VBAP GUI object-grab & position