vignette filter is not centered

[Michael Koch]

The vignette effect isn't centered in the frame, if the default values for x0, y0 are used:

ffmpeg -f lavfi -i color=white:size=8x8 -vf vignette=PI/4 -frames 1 -y out.png

It would be better to use (w-1)/2 and (h-1)/2 as default values:

ffmpeg -f lavfi -i color=white:size=8x8 -vf vignette=PI/4:x0=(w-1)/2:y0=(h-1)/2 -frames 1 -y out.png