Streams hold a reference to the source, but this reference should not be taken until `Start()` is called because freeing the media source depends on release in `Shutdown()` of the stream's reference to the source. We could create the streams in `media_source_create()` and take source refs for them in `Start()`, but that's potentially confusing and fragile.
Streams can persist after the media source is destroyed. In that case, they cannot access the source object and it should be released and set to null. Windows behaviour is to release references to the source in `Shutdown()`. If we don't do this and a buggy app were to leak a stream object, the media source object would also leak and `wg_parser_destroy()` would not be called.
-- v7: winegstreamer: Release stream references to the media source in Shutdown(). winegstreamer: Do not create MF stream objects until the media source is started. mfplat/tests: Test video stream release after media source release. mfplat/tests: Add more media source refcount checks.