[Feature request] Add ability to choose the visualization channel manually for PortAudio

[MurzNN]

Now for the PortAudio backend, there is no ability to choose the visualization channel manually, and by default, panon visualizes the microphone input. And I can't find a way to change it to visualize my output (speakers).
So, will be good to have ability to manually choose the visualization channel.

[TheOverpassArsonist]

for what it's worth this can be fixed temporarily with a tool like qjackctl. (or any other audio router) I can't pretend to know how to go about a more persistent fix, but depending on your uptime high-score it might be good enough for your usecase.