audio_resample: fix mapping of mono to single channel (#593)

audio_resample: fix mapping of mono to single channel (#593)

* audio_resample: fix mapping of mono to single channel

libav's mixer code can't map single channel layouts to other single
channel layouts. And we were asking it to map e.g. front left to front
center because out MONO mixdown == libav front center.  So when we
request MONO and the source is any single channel, change our output
layout to match the input.

fixes
https://forum.handbrake.fr/viewtopic.php?f=12&t=36016&sid=c5993fa7375792a940152c8adda19a54

* Incorporate Rodeo's suggestions

* fix grammar