decodebin3: Don't output bogus GST_MESSAGE_STREAMS_SELECTED

decodebin3: Don't output bogus GST_MESSAGE_STREAMS_SELECTED

When `is_selection_done` is called, it checks that all the requested streams are
present in the active stream list ...

... except there could very well be a (about to be removed) stream from the
previous selection present.

Therefore filter the list of streams we add to the message by the streams which
are actually requested.

Fixes issues when switching between different stream types (ex: video-only to
audio-only).

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3457>