This adds a RecordArea method to org.gnome.Mutter.ScreenCast. The intended user is a new screen cast D-Bus service in GNOME Shell replacing the built in one.

In short, it works by painting the stage given a calculated scale onto an offscreen framebuffer. The offscreen framebuffer is either directly from PipeWire or used as an intermediate buffer before copying its content to a PipeWire handled CPU memory buffer.