vnc: add alpha cursor support

vnc: add alpha cursor support

There is a new vnc extension for cursors with an alpha channel.  Use
it if supported by the vnc client, prefer it over the "rich cursor"
extension which supports only a bitmask for transparency.

This is a visible improvement especially on modern desktops which
actually use the alpha channel when defining cursors.

https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#cursor-with-alpha-pseudo-encoding

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20201208115737.18581-7-kraxel@redhat.com