transport: Add parsers before decoders and after encoders

This is in general required in 1.x and in specific if different stream formats
or parsed information are needed. For us the only format that has these
restrictions is H264 (and H265/HEVC later, or AAC if ever added).
transport: Don't shadow local variables
transport: Move valve from before the decoder to afterwards

If we have the valve before the decoder we will drop all
encoded frames until the stream is linked, including the
initial keyframe usually. So we have to request a new
keyframe which might or might not work but in any case
will take some extra time and delay the initial setup
of the pipeline.