Xpra: Ticket #721: openh264 decoder

See https://github.com/cisco/openh264 as an alternative to the ffmpeg based decoder. Limitations (from what I can see on their home page):

Sun, 26 Nov 2017 14:28:22 GMT - Antoine Martin: status, milestone changed; resolution set

commits seem to have dried up, I am betting on vp9 and AV1 (#1668)

Fri, 09 Mar 2018 17:59:15 GMT - urzds:

This is the H.264 de-/encoder that is available on e.g. Fedora and RHEL, where x264 is not available.

Sat, 10 Mar 2018 02:23:51 GMT - Antoine Martin:

The performance of this codec is nowhere near enough for us to use.

Our repository provides packages with the x264 encoder and the ffmpeg decoder for both centos (RHEL) and Fedora.

Sat, 23 Jan 2021 05:04:01 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/721