warholize: can use `-fx` instead of `-clut`

The `-clut` option requires ImageMagick 6.3.0 or higher; the ImageMagick manual 
suggests the use of the 
following `-fx` incantation for older ImageMagick versions:

    convert grayscale.jpg lut.miff -fx 'v.p{u*v.w,0}' colorized.jpg

The "warholize" tutorial could take advantage of this to enhance compatibility;
I think it should at least mention this as an alternative to `-clut`.

Original issue reported on code.google.com by riccardo.murri@gmail.com on 15 Nov 2012 at 11:15

abhilekhsingh / gc3pie

warholize: can use `-fx` instead of `-clut` #362