aggregator: Actually handle NEED_DATA return from update_src_caps()
The documentation says that this allows the subclass to signal that it needs more data before it can decide on caps, so let's actually implement it that way.
CC @meh @thiblahute
The documentation says that this allows the subclass to signal that it needs more data before it can decide on caps, so let's actually implement it that way.
CC @meh @thiblahute