Vignetting effect

[olivierdalang]

Hi !

Do you think we could implement a vignetting filter ? I think it's a really nice filter, very useful when illustrating blog posts for example.

I found a reasonable implementation here : http://php.net/manual/fr/function.imagefilter.php#109809 so it would be quite easy to add to the library. If it fits the scope of the library (which I think, since there are other nice&easy filters like sepia), I'd be happy to make a PR.