audioconvert: never do mixing for 1->1 channel conversions

audioconvert: never do mixing for 1->1 channel conversions

MONO and NONE position are the same, for example, but in
general there isn't much to do here for such a conversion.

Fixes problem in audioconvert, which would end up using
a mixmatrix when converting between different mono format
because it thinks MONO positioning is different from
unpositioned channels, which is not the case in this
special case. The mixmatrix would end up being 0.0 so
audioconvert would convert to silence samples.

https://bugzilla.gnome.org/show_bug.cgi?id=724509