GStreamer: Cyclic reference dependency between the objects and associated callbacks

[tharkum]

Need to break cyclic reference dependency between Player/Source and their associated callbacks to release all associated gst resources from PlayerInner , ServoSrc.

https://github.com/servo/media/blob/main/backends/gstreamer/player.rs#L580
PlayerInner and SignalAdapter callbacks (strong references to owner of SignalAdapter)

https://github.com/servo/media/blob/main/backends/gstreamer/player.rs#L682
ServoSrc and its callbacks (strong references to itself)