rtpbasedepayload: set default buffer offset based on RTP timestamp

Set a default offset on buffers: the delta between this buffer's RTP timestamp and the first RTP timestamp. This can be used when forwarding (depayloading/repayloading) media for codecs without specific offset logic in their payloader subclasses (e.g. Opus). The default offset allows the symmetric payloader to reconstruct the original RTP timestamp deltas for its output RTP timestamps, rather than having to rely on the (potentially altered) buffer PTS.

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