Make it possible to draw text with RHI

With Qt6 and OpenGL you can draw text with QOpenGLPaintDevice and QPainter. There does not seem to be any way to draw text with Qt6 and RHI. We are considering porting a software from OpenGL to RHI and this is a show stopper.

One way to address this would be to add a class QRhiPaintDevice. But there may be better ways, such as adding a function

    QRhiCommandBuffer::drawText(const QString& text, const QFont& font, ... );