UI: Fix display of mono source with surround output

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A mono input with surround output is upmixed by swresampler by routing
it to FC (front center) by default which corresponds to third channel
for 3.0, 4.0, 4.1, 5.0, 5.1, 7.1. (The other channels are muted.)
But the volume meters are set to show the first channel for mono sources.
Therefore although audio is processed, it would not show on the meter.
This fixes this display issue.

Closes obsproject/obs-studio#1540

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