VBAP panner UI tweaks:

* fix mouse-grab of sentinel
* make GUI more hemisphere like (circles at latitude)
* change alpha slightly to show signal overlap
take process lock when adding processors:

fixes possible crash if a processor modifies port-count

1. a processor is inserted and activated with processor-lock held
2. only after that the process_lock() is taken, configure_processors() is called which reconfigures-IO

BUT if the processor that is inserted changes the channel count AND audio is processed before IOs are reconfigured
 -> possible crash (invalid port-buffers)

To reproduce: Bus1 (2in, 3out), Bus2 (2in, 3out)
- add a send from Bus1 to Bus2,
- then add a processor to Bus1, just before the send which
  increases the channel-count to 4 -> occasional crash or assert.