videoaggregator: Don't require any pads to be configured for negotiating source pad caps
This is not actually required (anymore?). Source pad caps can be negotiated at any time regardless of any configured (or existing) sink pads and videoaggregator comes up with some fixated caps based on the downstream caps.
Subclasses can override this behaviour as needed by overriding update_src_caps().
CC @thiblahute @meh