screen-cast-src: Allow negotiating the framerate

The framerate for screen cast sources was set to variable within 1 FPS and the framerate of the monitor being screen casted. This meant that if the sink didn't match the framerate (e.g. had a lower max framerate), the formats would not match and a stream would not be established.

Allow letting the sink clamp the framerate range by setting it as 'unset', allowing it to be negotiated.