Text underline position is incorrect when using the DirectWrite font engine

QFontEngineDirectWrite doesn't override QFontEngine::underlinePosition to return the font's correct underline position, meaning that underlining looks very bad when using the DirectWrite engine.

To see this, build and run the attached test app. The underlining looks great with the GDI engine used by default. Now switch on the 'DirectWrite' checkbox. Observe that the underlining moves far too close to the text. The font is Times New Roman.

The correct underline poisition is available from the DWRITE_FONT_METRICS structure and could be cached in QFontEngineDirectWrite::collectMetrics.