V4L2 loopback camera has warped colors

When using V4L2 loopback to create a capture device with yuv420p pixel format, the video frame colors are skewed.

Steps to reproduce:

1. Create a loopback capture device with the v4l2loopback kernel module: sudo modprobe v4l2loopback video_nr5
2. Play test stream over loopback device using ffmpeg: ffmpeg -re -f lavfi -i testsrc -vf format=yuv420p -f v4l2 /dev/video5
3. Preview loopback camera using the declarative-camera or camera examples

Expected

• Test video is shown as normal

Actual

• Video brightness (Y-plane) is shown normally with colors (UV) being warped.
• Using ffplay or VLC for preview shows correct colors