Smart rendering has been broken since, mostly forever, but some code
was there pretending it was supported... let's try to stop pretending.

We now keep track of the smart rendering state in the timeline, track
and sources to be able to:

 * tell decodebin to stop plugging more (decoding elements) as soon as
   downstream supports the format.

 * avoid plugging converters after the source element when smart
   rendering.

Part-of: <!198>