deb: switch back to system libraries for ffmpeg on newer distros

[Antoine Martin]

Newer Debian / Ubuntu distros provide ffmpeg libraries that are recent enough for us to use:

• ​stretch: 3.2.9 (maybe)
• ​buster: 3.4.1
• ​artful: 3.3.4
• ​bionic: 3.4.1

We should deprecate our private package (#1256) for those distros and ensure we build against the system version.
This may require using Conflicts and Replaces tags to ensure the package is not installed.

[Antoine Martin]

Done for "bionic" in r18106.

TODO:

• test install with bionic alpha
• test upgrade with artful

[Antoine Martin]

r18222: buster should now upgrade and remove the old ffmpeg-xpra

Let's not deal with stretch and artful, leaving it to the distro upgrade to make the transition to the new dependencies.

[Antoine Martin]

This will do for now, hopefully ffmpeg 4.0 won't force us to use functions that aren't available in 3.4.x (see #1772)

[Antoine Martin]

Had to clean the build chroot and remove ffmpeg-xpra to ensure it doesn't get picked up by the build file because then the DEB both depends and conflicts with the ffmpeg package!

[Antoine Martin]

Removed completely in v4: r23978.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1751