GstGLVideoItem is required to render input video in the overlay's qml.
And currently qmlgloverlay requires to set this GstGLVideoItem to its widget property.
Instead of fetching GstGLVideoItem from the overlay's root object (root-item prop),
and setting it back as a widget (widget prop),
proposing to use found GstGLVideoItem in the current object hierarchy (passed in qml-scene) by default.
Also useful in Python, which solves the issue when casting gpointer <=> QQuickItem* is required.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/919>
