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.
18 jobs for !1380 with vaapioverlay-optimize in 51 seconds (queued for 2 seconds)
merge request