Previously the different decoders would discard errounous GstFlowReturns coming
from downstream. Now we properly return these further upstream so that we
properly error out on eg. negotiation problems.
we now no longer try to get the GstVdpDevice from downstream since it in
practice didn't give us anything and complicates the code alot. Nevertheless if device
distribution should be done there's probably a lot better ways to do it.
