pulsesink: Add support for custom downmix matrizes

Submitted by Ross Burton `@ross`

Playing a AC3 movie in Totem with PulseAudio results in bad mixing, meaning I can't hear the talking over the background noise of the rain.

thaytan helped me debug this: PulseAudio takes all 5 channels and then mixes them badly, when the mixing should happen in the gstreamer pipeline.