[FR] Make the window size/position available for Xdp.Session

[KelvinNovais]

When making a screencast of a window, the size and position of the window should be informed.

Currently, calling xdp_session_get_streams returns only the size of the whole stream. This information is not meaningful for a window stream.

See the output below:

It's the stream of a single window, but the monitor size is considered as a black background.

By calling xdp_session_get_streams, it returns [(uint32 92, {'id': <'0'>, 'source_type': <uint32 2>, 'size': <(1920, 1080)>})]. Notice that the size value corresponds to the whole monitor. Since I'm interested of outputting a window, I have to figure out its size and position in order to crop it.

Also, the window can vary its size during its lifetime. This change should be notified.