Enable Pan Link Option For Four Main Busses

Sending channels to buxxes and matrixes allows for unlinking the send’s pan from the overall channel pan. However, with the four main busses, the only option is to follow the main channel pan. There should be an option to link/unlink the send to Main pan from the overall channel pan.

An example of this usage might be for panning all instruments to the center in the house PA to equal room coverage, but then panning slightly for a stereo broadcast mix.