WIP: qtdemux: Reusing streams when switching encryption status.

In fragmented MP4, when moving from an encrypted fragment to an unencrypted one, we should signal the stream as still being an encrypted one. If this does not happen, the pipeline will have to reconfigure itself, removing the original decryptor and instead using a plain h264 decoder. Decodebin currently does not support this, and it's arguable whether it needs to. Instead, the approach taken here is that once a stream has been detected as encrypted, its caps will reflect that forever more.

Clear buffers should be handled correctly by decrytpors, since they will notice the lack of a GstProtectionMeta associated with them.