textoverlay: Query downstream caps for checking if caps features are supported, not just accept-caps

accept-caps is not recursive and might stop at the next downstream element,
while caps queries are generally recursive. The next element might accept any
capsfeatures we want, but that doesn't mean that further downstream it will
also work.

Additionally for the future:
We should probably check if downstream *prefers* the
overlay meta, and only enforce usage of it if we can't handle
the format ourselves and thus would have to drop the overlays.
Otherwise we should prefer what downstream wants here.