The case at hand was 5.1 -> fl-fr-fc-lfe-na-na (apparently triggered by
ALSA). That means only the NA channels have to be cleared, but the
result was actually that fc and lfe were cleared. This is due to a
simple regression in the reorder code, which quite obviously got the
index of the first NA channel wrong.
Move it from af_lavrresample.c to a new aconverter.c file, which is
independent from the filter chain code. It also doesn't use mp_audio,
and thus has no GPL dependencies.
Preparation for later commits. Not particularly well tested, so have
fun.
