rtpulpfecdec: Implement Microsoft extensions

Microsoft has a slightly difference variant. In particular, they ignore
RTP header extensions when doing FEC and the sequence number is stored as an offset
instead of as the complete number. They also add an extension to the FEC header itself.

Only the XOR variant is implemented. The MS-H264PF document hints at a Reed-Solomon
based variant, but doesn't explain it.

Ref: MS-H264PF https://docs.microsoft.com/en-us/openspecs/office_protocols/ms-h264pf/ab2ee1d8-ce1b-46f3-b128-9751256b6f18
18 jobs for rtpulpfec-msft-extension in 81 minutes and 52 seconds (queued for 2 seconds)
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  Preparation
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