Changing fallback fonts in running application can cause random text in Qt Quick

Currently, if you change the fallback font for a given writing system while there is text showing in that writing system (so that a fallback is already loaded and cached) and then show some new text in that writing system, we might end up with random glyphs in the output.

One possibility is that we end up clearing the cached font in the main thread so the glyphs may refer to the correct font, but in Qt Quick it will still resolve to a cached font engine because the main font is the same.

So it's possible we need to make sure fallback fonts are reloaded after a clear has happened somehow.

For now, the fallback fonts have to be decided up front and cannot be changed while the Qt Quick scene is showing.