winrt: enable cross-platform high DPI scaling

Task-number: QTBUG-46615
Change-Id: I7f75bc7da35b9330753130338a06feb49533061c
Reviewed-by: Andrew Knight <andrew.knight@intopalo.com>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@theqtcompany.com>

src/plugins/platforms/winrt/qwinrtscreen.cpp

@@ -635,6 +635,12 @@ QDpi QWinRTScreen::logicalDpi() const
     return QDpi(d->logicalDpi, d->logicalDpi);
 }
 
+qreal QWinRTScreen::pixelDensity() const
+{
+    Q_D(const QWinRTScreen);
+    return qRound(d->logicalDpi / 96);
+}
+
 qreal QWinRTScreen::scaleFactor() const
 {
     Q_D(const QWinRTScreen);

src/plugins/platforms/winrt/qwinrtscreen.h

@@ -93,6 +93,7 @@ public:
     QImage::Format format() const Q_DECL_OVERRIDE;
     QSizeF physicalSize() const Q_DECL_OVERRIDE;
     QDpi logicalDpi() const Q_DECL_OVERRIDE;
+    qreal pixelDensity() const Q_DECL_OVERRIDE;
     qreal scaleFactor() const;
     QPlatformCursor *cursor() const Q_DECL_OVERRIDE;
     Qt::KeyboardModifiers keyboardModifiers() const;