This file used to render fine: https://de.wikipedia.org/wiki/Datei:BuickLogo_silber.svg and now doesn't with Thumbor. It has a large amount of entities before the SVG namespace appears, which is probably why that doesn't get picked up, it's probably beyond the length limit of the buffer excerpt.

Maybe we should relax our is_svg check to just not check the namespace presence and try anything that starts with <?xml or <?svg ? In which case the SVG engine should be one of the last to be tried in the list of engines.