lavfi/showspectrum: display multiple channels in separate row

The showspectrum filter gets multiple channel (any count) support.

Signed-off-by: Rudolf Polzer <divverent@xonotic.org>
wmavoice: silence may be used uninitialized warnings

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
flacdec: silence several "warning: X may be used uninitialized in this function"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>