Allow mapping multiple outputs to an input

[danngreen]

The outputs would mix together equally. PatchPlayer could handle this efficiently when iterating the cables.

This was introduced in a recent VCV Rack version.

I'm thinking the patch format could stay the same, but to use this feature a patch would have
multiple InternalCable entries, each with a different out jack, but at least one common entry in the vector of ins.
When loading the patch, PatchPlayer would need to separate the multi-out-mix connections to a new set of connections that are processed differently. We would also need to cache the output from each module's get_output so we don't call it twice (in the case of a multi-out split overlapping a multi-in split).