This allows user to be in control of when scaling happens in the pipeline.
It can be plugged as an effect and scaling will be forced to happen in
the effect instead of in the compositor.
Without this, it would not be possible for users to, for example,
crop a video source before scaling to the target source size.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5735>
The previous code was storing container children in reverse
addition order, this was mitigated by the fact that track elements
were also stored in reverse order, thus restoring the original
order, but it seems more consistent to preserve order throughout,
the extra cost of append operations is negligible.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1549>
