QtWebEngine content does not scale according to DPI

Unlike other Qt UI elements, the content rendered by WebEngine does not scale according to the Xorg DPI. Only explicitly setting QT_SCALE_FACTOR scales the content, but it scales the entire UI (which was already scaled according to DPI).

Common workarounds in web browsers involve changing the default zoom level, but that has its own problems (delays due to waiting for the whole page to load, improper rendering etc).

Issue originally reported for qutebrowser in [1], but the issue affects other browsers as well.

[1] https://github.com/qutebrowser/qutebrowser/issues/4494#issuecomment-473067602