VBAP: Fix stairstepping during modulation

[nhthn]

this fixes #122 and replaces #123. i'm kicking myself because i delayed on this an entire year and only just now realized how simple the bug was.

the problem was a failure to increment the channel gains by the slopes in the nova-simd version of the code.

~array = VBAPSpeakerArray(2, [-30, 30, 0, -110, 110]);
~buf = ~array.loadToBuffer;
{ VBAP.ar(5, DC.ar(1), ~buf.bufnum, Line.kr(0, 360, 0.05), 1.0, 0.0) }.plot(0.05);

before:

after:

[muellmusik]

This looks good, Nathan

[muellmusik]

I'll combine with the VBAP 1.0.3.2 stuff etc. when I have time later. A bit bigger job...