aggregator: don't forward reconfigure events

Those will cause us to renegotiate at the next aggregate cycle, and while at that point we may decide to reconfigure upstream branches (in practice we don't as this is inherently racy, and that's the reason why mixer subclasses perform conversion internally), we certainly don't want to just forward the event willy-nilly to all our sinkpads.

An actual issue this is fixing is when caps downstream of a compositor are changed at every samples-selected signal emission, for the purpose of interpolating the output geometry, and the compositor has a non-zero latency, the reconfigure events were forwarded to basesrc, which triggered an allocation query, which in turn caused aggregator to have to drain (thus not being able to queue frames), leading to disastrous effects (choppy output as compositor couldn't consume frames fast enough, the higher the latency the choppier the output)