QImageWriter::write() always uses inappropriately low compression when writing to webp format

When asked to save an image to .webp format (even if it is already a .webp image), QImageWriter uses the lowest possible compression by default, resulting in images that, while very small, are unusably bad. qimagewriter::setQuality(100) seems to have no effect, either.

This issue was discovered when debugging an issue in KDE Gwenview. The code in question is here: https://cgit.kde.org/gwenview.git/tree/lib/document/documentloadedimpl.cpp#n81

I've attached a sample jpeg image and the ugly webp image that it becomes when QImageWriter is used to convert it.