videofilter: Allow to filter only areas indicated by GstVideoRegionOfInterestMeta

As a follow up of the issue https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/1253, there is interest on modifying gstvideofilter to optionally filter just regions of interests.

I quote @thiblahute 's proposal:

The goal is to have GstVideoFilter handle the 'cropped view of the frames' for all its subclasses, calling GstVideoFilter:transform_frame_ip with the frame that has been mapped only for the region where the effect has to be applied. For a same frame with several ROI (selectable by the user), VideoFilter::transform_frame_ip could be called several time for a same frame with the different regions mapped (the ordering of the ROI will matter when there are intersections.. but we should ignore that).

For VideoFilter::transform_frame iself, and as a second step (as I think as a first step we should just support filter that only implement VideoFilter::transform_frame_ip), we should have VideoFilter copy the input buffer, crop it, call transform_frame with a cropped version of the input/output buffers, then blend the buffer back into the original full frame.

NOTE: This feature should be opt-in by subclasses of VideoFilter and we should see how that could work for hw accelerated effects.

Also from @meh 's comment:

some alignment might be needed, for example to deal with chroma subsampling, but that could be clearly documented.

If such an approach is possible, we could then apply any existing effect to regions of interest. The API should also allow specifying what ROI types we want the effect applied to.