libavformat: Move avc mp4 startcode parsing to a shared file

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpenc: HEVC/H.265 support

Signed-off-by: Martin Storsjö <martin@martin.st>
rtsp: Clear the session id on redirects

This fixes handling redirects in case the server provided a session
id within the redirect reply.

Signed-off-by: Martin Storsjö <martin@martin.st>