decodebin3/playbin3: Fix playback failures
decodebin3: Remove failing stream from selection
If no decoder is present for a given stream, remove it from the requested selection.
Fixes #1581 (closed) and #1662 (closed)
decodebin3: Don't intercept queries if no parsebin present
If we don't use a parsebin, we forward the queries as-is from upstream. There is no reconfiguration possible within identity.
decodebin3: Don't insert parsebin if input is already parsed
This is a temporary workaround until we find a generic solution to indicate that a stream has already been "parsed".