videoaggregator: Guarantee that the output format is supported

In the case videoaggregator is set as allowing format conversions, and as we convert only on the sinkpads, we should ensure that the chosen format is usable by the subclass. This in turns implies that the format is usable on the srcpad.

When doing conversion any format can be used on the sinkpads, and this is the only way that we can avoid race conditions during renegotiations so we can not change that fact, we just need to ensure that the chosen intermediary format is usable, which was not actually ensured before that patch.

Fixes #834 (closed)