[magick-users] -level and getting proper values for it

[duc.sequere.aut.de.via.decede at imagemagick.org]

> convert image -format "%[fx:image.minima]" info:

For about 5 minutes we attempted to have fx:image.mean to return the mean
for all the channels but that caused a problem with the FX parser.  So
intead it returns the mean for the red/cyan channel unless you do what Fred
suggests:

  convert image -channel Green -separate -format "%[fx:100*image.mean]" info: