This commit adds basic screen casting and remote desktoping
functionalty. This works by exposing two D-Bus API services:
org.gnome.Mutter.ScreenCast and org.gnome.Mutter.RemoteDesktop.

The remote desktop API is used to create remote desktop sessions. For
each session, a D-Bus object is created, and an application can manage
the session by sending messages to the session object. A remote desktop
session the user to emit input events using the D-Bus methods on the
session object. To get framebuffer content, the application should
create an associated screen cast session.

The screen cast API is used to create screen cast sessions. One can so
far either create stand-alone screen cast sessions, or a screen cast
session associated with a remote desktop session. A remote desktop
associated screen cast session is managed by the remote desktop session.

So far only remote desktop managed screen cast sessions are implemented.

Each screen cast session may have one or more streams. A screen cast
stream is a stream of buffers of some part of the compositor content.
So far API exists for creating streams of monitors and windows, but
only monitor streams are implemented.

When a screen cast session is started, the one PipeWire stream is
created for each screen cast stream created for the session. When this
has happened, a PipeWireStreamAdded signal is emitted on the stream
object, passing a unique identifier. The application may use this
identifier to find the associated stream being advertised by the
PipeWire daemon.

The remote desktop and screen cast functionality must be explicitly be
enabled at ./configure time by passing --enable-remote-desktop to
./configure. Doing this will build both screen cast and remote desktop
support.

To actually enable the screen casting and remote desktop, the user must
enable the experimental feature. See
org.gnome.mutter.experimental-features.

https://bugzilla.gnome.org/show_bug.cgi?id=784199