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
owr: Constify some static arrays
transport: Use the highest ranked, working decoder/encoder for VP8/H264 from the registry

This allows us to use existing codecs on the user's system, especially it
allows to use hardware codecs and removes a dependency on openh264.