Collect random image processing into a utility (pseudo) module; shields from presence of Image::Magick
get image sizing from LaTeXML::Util::Image and be careful about pixels, points and rescalings
get scaling from LaTeXML::Util::Image and fail gracefully if Image::Magick is needed but not found
Remove Image::Magick as a requirement; add Image::Size as a requirement
document new requirement Image::Size