decodebin3: Avoid parsebin usage if not needed

decodebin3: Avoid parsebin usage if not needed

If the incoming streams are already parsed, there is no need to add yet-another
parsebin to process it *IF* that stream is compatible with a decoder or the
decodebin3 output caps.

This only applies if all the following conditions are met:
* The incoming stream can *NOT* do pull-based scheduling
* The incoming stream provides a `GstStream` and `GstStreamCollection`
* The caps are compatible with either the decodebin3 output caps or a decoder
  input

If all those conditions are met, a identity element is used instead of a
parsebin element and the same code paths are taken.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2784>