Allow for multiple output nodes

[Lubba-64]

This is a quality of life idea that I came up with. Currently you can only have one output node
that transports the sound from the internal engine to the VST plugin. Multiple nodes will not
crash the VST, but will lead to undefined behavior (thankfully not UB that’s memory unsafe, just
that we haven’t defined what happens in that case). The proposed fix is defining a behavior of
just mixing the inputs to each output node together which should be sufficient. This should not
be too hard to implement, but might take extra time so I feel like 1 hour is a comfortable
estimate.