REG: Warnings when drawing text with DirectWrite

After bb67c3b68b00fb2075a971f4f0d58e1867ef7128, Qt Widgets applications may see warnings like this:

QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::end: Painter not active, aborted

This seems to be because we in certain cases before would return a blank image consisting only of margins, but since we now respect the glyph margins in the font engine, we return an invalid image instead.