Draft: uridecodebin: Do not flush previous source when doing instant change

Doing an instant URI change aims at changing the source asap and we should still try to make the transition smooth. Flushing the current source will leads to uridecodebin3 not outputing anything while the new URI is getting ready. Removing that flush will make the transition smooth as we will keep using the previous source meanwhile.

I tried to discuss that on IRC but we didn't reach any conclusion. In my case I have simple RTP streams and I need to be able to smoothly switch between them. If we flush the current source while the new one is not ready (it needs to wait for keyframe before being able to do anything) the output stream freezes, keeping using the old one meanwhile seem more appropriate to me.

cc @bilboed

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