The attached patch adds multichannel support to GStreamer.

It adds a set of helper functions to the libgstaudio.so plugin which handle
obtaining channel lists in a structure, setting channel lists and fixating
channels to default values. The last is useful for the generic case, but does
not work in specific cases (separating channels over different elements and so
on). also, the patch adds support for handling multiple channels to the faad,
a52dec, dtsdec, audioscale and alsasink plugins.

The multichannel support consists of an enum "channel-positions" that is added
to a GstStructure. In 0.8.x, this exists in addition to the channels=X option.
channels=1/2 (which implies mono/stereo) don't need this property, all other
values do. After 0.8.x, the channels=X property is to be removed in favour of
this list. The enums are possible channel positions. Some are mututally
exclusive, but the documentation and the helper funtions mostly take care of that.

It works for me, but it is somewhat slow during autoplugging. Starting a song in
RB takes 5-7 seconds. :-(.

Still TODO: volume, audioscale, faac, ffenc_ac3, vorbisenc/dec, several
muxers/demuxers (matroska and avi can contain multichannel PCM sound, qt
probably too).