Try to organise this better to make it clear which dependencies are
ours, and which we've gotten just because Xvnc is based on xorg-server.

Also update the dependency list to the current state of things.

This is what upstream does, so let's add it in case some dependency is
added implicitly by the Debian build tools somewhere.

This is done for us, so stop having explicit lists that we forget to
update properly.

Follow the format upstream Debian/Ubuntu uses.

It is generally pulled in as a requirement from GnuTLS, but let's be
explicitly now that we make use of nettle directly.

Linux implementation using ffmpeg

This was removed from Ubunut's X.org packages years ago, so drop it from
our packing as well.