This fixes issues with send-panner bypass whenever the
target bus input-count is different from the send's channel count.
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Previously, when the aux-send panner was bypassed, data was
copied using BufferSet::read_from().
This sets the channel count of the output buffer set (here: mixbufs)
to match the input (here: bufs).
e.g. mono to stereo, "1 in -> 2 out" out was changed to "1 in -> 1 out".
Un-bypassing the panner later does not reconfigure the I/O.
Mixbufs remained mono, and PannerShell::run() "1 in -> 1 out"
does nothing. The panner was effectively not functional.
5b113c9c5b