Our XMP handling is spec-compliant while imagemagick's isn't

according to https://exiftool.org/forum/index.php?topic=7509.msg38182#msg38182 imagemagick writes XMP profile to PNG with keyword `Raw profile type xmp` and has other encoding differences like writing it to zTXt instead of iTXt. https://sourceforge.net/p/graphicsmagick/bugs/682/ also mentions this imagemagick behavior I have not tested this locally yet.