[Bug 784010] Compositor produces broken video (with green stripes) when compositing BGRA and I420 that has been scaled

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=784010

--- Comment #1 from Sean-Der <sean at siobud.com> ---
I fixed this by setting the pixel-aspect-ratio (example pipeline below) while
debugging I noticed that the videoconversion in the compositor was changing the
height of the video

video/x-raw, format=(string)I420, width=(int)1200, height=(int)680,
interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)136/135,
chroma-site=(string)mpeg2, colorimetry=(string)bt709, framerate=(fraction)30/1
to
 video/x-raw, format=(string)BGRA, width=(int)1208, height=(int)680,
interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1,
colorimetry=(string)sRGB, framerate=(fraction)30/1

It was using the pixel-aspect-ratio, so by forcing it after the video scale it
keeps the height/width (and the green lines go away)

gst-launch-1.0 videotestsrc ! video/x-raw, width=10, height=10, format=BGRA !
compositor name=c ! videoconvert ! ximagesink \
  videotestsrc ! video/x-raw, format=I420, width=1280, height=720 ! videoscale
add-borders=false ! video/x-raw, width=1200, height=680, pixel-aspect-ratio=1/1
! c. \

I would love to take this bug on (so I can understand the GstVideoAggregator
and video convert code more) but would need someone to lead me in the right
direction. Or maybe the behavior right now is fine, and I just don't fully
understand

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