Update encoding loop to recommended algorithm; add drain loop.

[dbruning]

#191 mentions that the code in H264VideoStreamEncoder.cs isn't a "correct" encoding loop.
I implemented the recommended loop from https://ffmpeg.org/doxygen/4.1/group__lavc__encdec.html
It's a bit more verbose than it needs to be, but explains the algorithm as it goes.

I think the original H264VideoStreamEncoder.cs had 2 problems:

• the output would have missing (or maybe duplicated) frames at the start, because it kept sending the same frame in rather than sending it once and then reading packets (if any) until EAGAIN
• the output would have missing frames at the end, because it didn't drain.

Thanks for your great work in this project!

[Ruslan-B]

Thank you for the contribution. I might tweak it later on but only with respect to coding standards. I keep auto generation running but lacking time to observe changes in documentation or examples. I had to admit FFmpeg API going to be more and more complex over years.