QPainter drawText and QFontMetrics not thread safe

Text rendering and accessing font information through QFontMetrics is unstable when done on multiple threads.

The attached project is a simple demo that renders text and an image of text in a QQuickView. The image of the text is first rendered on a separate thread to a new image before displayed. Both the QML text and the rendered text image will show severe rendering artifacts. Stop the text generation on the other thread and the issues goes away.

This may to be due to the management of font resources in the Windows font engine plugin.