Support Imagemagick 7

[Renarde-dev]

Imagemagick 7 introduce breaking changes :

• convert is deprecated in favor of magick (#2366)
• -coalesce is not implied when formating webp which break compression

webp conversion should be handle like this :

magick "input.webp" -coalesce -filter Lanczos -resize 120x120! -strip -quality 70 -interlace line -sampling-factor 4:2:2 "output.webp"

Command detection should be handled with

if ('command -v magick') {
    shell_exec('magick ....');
} else {
    shell_exec('convert ....');
}

[Renarde-dev]

Branch : https://github.com/Piwigo/Piwigo/tree/issue-2367-imagemagick7

[plegall]

branch merged