compositor: Checkboard pattern not square for UYVY

Compare the following two pipelines

Not square (twice as wide than high)

gst-launch-1.0 videotestsrc ! video/x-raw,format=UYVY ! compositor sink_0::alpha=0 ! videoconvert ! autovideosink

Square:

gst-launch-1.0 videotestsrc ! video/x-raw,format=ARGB ! compositor sink_0::alpha=0 ! videoconvert ! autovideosink