[gstreamer] pixel-aspect-ratio keeps always scales width

when scaling a video with a pixel-aspect-ratio, gstreamer will always keep the pixel height constant, but will scale the width.
the ffmpeg backend however will scale the height, if the pixel aspect ration is less than 1/1.

the gstreamer behaviour is hardcoded do try to keep the height "because of interlacing": https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-base/gst/videoconvertscale/gstvideoconvertscale.c?ref_type=heads#L1587