msdk: manage child sessions on parent GstMsdkContext

Sometimes parent context is released before its children get released.
In this case MFXClose of parent session fails.

To make sure that child sessions are closed before closing a parent
session,
Parent context needs to manage child sessions and close them first when
it's released.

https://bugzilla.gnome.org/show_bug.cgi?id=793412
1 file changed