audioaggregator: fix channel-mask negotiation

When outputting more than two channels, a channel-mask has to be
specified in the output caps.

We follow the same heuristic as other cases, when downstream
does not specify a channel-mask, we use that of the first
configured pad, and if there was none we generate a fallback
mask.

https://bugzilla.gnome.org/show_bug.cgi?id=794257
1 file changed