compositor: Checkboard background is yellow/blue with BGRx / RGBx

# Yellow
gst-launch-1.0 videotestsrc ! video/x-raw,format=BGRx ! compositor sink_0::alpha=0 ! videoconvert ! autovideosink
# Blue
gst-launch-1.0 videotestsrc ! video/x-raw,format=RGBx ! compositor sink_0::alpha=0 ! videoconvert ! autovideosink