rtpmp4gdepay: Set duration on outgoing buffers
If we have constant duration buffers, set the duration on outgoing buffers, like rtpmp4adepay does. This fixes problems with (for example) muxers like mp4mux not writing the duration of the final sample into the index.
Similar to !6447 (merged) which set outgoing durations for the other MP4A depayloader rtpmp4adepay