Extract snapshots images are corrupted/gray

[PseudoResonance]

The extract snapshots docs show an example command:

bash -c "
while true; do
  mkdir -p $(dirname snapshots/$MTX_PATH)
  ffmpeg -i rtsp://localhost:8554/$MTX_PATH -frames:v 1 -update true -y snapshots/$MTX_PATH.jpg
  sleep 10
done"

But this command will probably fail with many streams, because it will most likely not extract a keyframe, and instead extract a P/B frame.

Simply for reference, I figured I would post a more complete command that will wait for a keyframe before extracting. I think it'd probably be nice to include some sort of notice about this on the documentation.

bash -c "
while true; do
  mkdir -p $(dirname snapshots/$MTX_PATH)
  ffmpeg -hwaccel auto -strict experimental -rtsp_transport tcp -skip_frame nokey -i rtsp://localhost:$RTSP_PORT/$MTX_PATH -an -sn -dn -c:v libwebp -q:v 100 -frames:v 1 -update true -y snapshots/$MTX_PATH.webp
  sleep 10
done"