videoaggregator: fix caps queries to allow proper renegotiation

videoaggregator: fix caps queries to allow proper renegotiation

When caps are already negotiated it should be possible to
select formats other than the one that was negotiated. If downstream
allows alpha video caps and it has already negotiated to a non-alpha
format, caps queries should still return the alpha caps as a possible
format as caps renegotiation can happen.

Includes tests (for compositor) to check that caps queries done after
a caps has been negotiated returns complete results

https://bugzilla.gnome.org/show_bug.cgi?id=757610