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

Submitted by Sean-Der

Description

Using the following pipeline

gst-launch-1.0 videotestsrc num-buffers=1 ! 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 ! video/x-raw, width=1200, height=680 ! c.

I get video that looks like http://i.imgur.com/WK9bh9d.png

If you change the format to I420 on the first videotestsrc this issue goes away. I use the videoscale outside the compositor for better performance (works is spread across more threads it seems) if I don't I peg a single core much quicker.