vaapioverlay: inline sinkpad scaling support

vaapioverlay: inline sinkpad scaling support

Often, users will need to scale inputs (e.g.
with vaapipostproc) before they are submitted
to the vaapioverlay.  However, this results in
multiple VPP passes/operations in the pipeline
which creates unnecessary process overhead.

This change allows for inputs to be submitted
at original scale to vaapioverlay with per-sinkpad
scale dimensions specified so they can be scaled
and blended/composited in a single VPP pass/operation
to avoid the unnecessary process overhead.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1380>