Merge remote-tracking branch 'remotes/kraxel/tags/ui-20200123-pull-request' into...

vnc: fix zlib compression artifacts.
ui: add "none" to -display help.

# gpg: Signature made Thu 23 Jan 2020 14:20:53 GMT
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>" [full]
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>" [full]
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>" [full]
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/ui-20200123-pull-request:
  ui/console: Display the 'none' backend in '-display help'
  vnc: prioritize ZRLE compression over ZLIB
  Revert "vnc: allow fall back to RAW encoding"

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>