Decoded RTP fixed header and payload (RFC 3550).
The csrc array's length is always versionFlags.csrcCount; on encode,
versionFlags.csrcCount must match csrc.length or the CSRC list will be
mis-sized. Header extensions (RFC 3550 §5.3.1) are not parsed — when
versionFlags.extension is set, the extension block is included verbatim
at the start of payload.
versionFlags: { version: number; padding: number; extension: number; csrcCount: number; }
markerPayloadType: { marker: number; payloadType: number; }
sequenceNumber: number
timestamp: number
ssrc: number
csrc: number[]