Actually set devicePixelRatio on copy.

Followup to 7b9d4531 - the second part refactors
to call setDevicePixelRatio but the call itself
was left out.

Change-Id: I3e36452603fe1d7d53fa1a74d87169efea1c2e78
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/gui/image/qpixmap_raster.cpp

@@ -244,6 +244,7 @@ QImage QRasterPlatformPixmap::toImage(const QRect &rect) const
         QImage newImage(image.scanLine(clipped.y()) + clipped.x() * (du / 8),
                       clipped.width(), clipped.height(),
                       image.bytesPerLine(), image.format());
+        newImage.setDevicePixelRatio(image.devicePixelRatio());
         return newImage;
     } else {
         return image.copy(clipped);