Screen sharing from Pidgin

[David Woodhouse]

I'm working on screen sharing. It's just another video screen, but the
GStreamer src element is (in my case) ximagesrc instead v4l2src.

I suspect the best way to handle it is to define a new media type
PURPLE_MEDIA_SEND_SCREEN. It's then left to the UI (e.g. Pidgin) to
register an appropriate src element according to the windowing system
in use (be it X11, Wayland, Windows, OSX, etc.), and to handle advanced
things like choosing whether to share the whole display, or just one
monitor, or just one window.

I expect Finch would render its own terminal to images :)

Any objections or better ideas? Any opinions on
PURPLE_MEDIA_RECV_SCREEN? I don't actually see the point as receiving
that as video is fine for now, and we're never likely to need a full-
duplex version of screen sharing. Maybe one day in the future we'll
want to handle remote control, and send mouse and key events from a
RECV_SCREEN media session to the remote end — but for now doing only
PURPLE_MEDIA_SEND_SCREEN seems best.

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