QDomImplementation::DropInvalidChars strips emoji and other non-BMP characters

Downstream bug: https://bugs.kde.org/show_bug.cgi?id=455255

We are loading the SVG XML by wrapping a QIODevice in QXmlInputSource, then loading it with QDomDocument::setContent. After calling QDomImplementation::setInvalidDataPolicy(QDomImplementation::DropInvalidChars), QDomDocument::setContent strips non-BMP chars encoded in UTF-8 in the XML.

QDomDocument is loading the file as UTF-16 code units. In fixedCharData, it checks the code units one by one, using QXmlUtils::isChar which rejects half surrogates. This will, of course, strip all non-BMP code points.