playbin3: Configure combiner on pad-added if needed
When collection is updated, decodebin3 exposes pad first and then streams-selected message is posted. The condition can cause a situation where playbin3 links non-existing combiner/playsink pads (since streams-selected is not posted yet) with new decodebin output pad. This commit will re-check selected/active streams condition on pad-added and reconfigure output if needed.