interoperability doc

[scottlamb]

Write up a doc about how Retina's implementation compares and interoperates with other RTSP clients and servers. Some points to mention:

• how it joins base URLs with control URLs. (As described in no control url #9, other implementations do various crazy things.)
• what the various policy knobs do and when they're necessary
• other camera-specific bugs it works around
• anything it expects that other RTSP libraries don't and why. (eg from packet follows marked packet with same timestamp #13 it cares more about the RTP marker flag and timestamps than ffmpeg does, and I think the advantage is trimming a frame of latency. but it seems we'll need at least need to relax this a little.)

Maybe a chart of different RTSP implementations' choices would be helpful. I also like to have links to the relevant parts of other open source libraries.