Avoid detaching pixmaps when it has not changed

setDevicePixelRatio is often called unconditionally even when the
devicePixelRatio matches the pixmap or image.

This causes a lot of unncessary detaches wasting some memory.

Change-Id: I27535b2b22312ec0edc9bdc00c99d322afb727c1
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/gui/image/qimage.cpp

@@ -1433,6 +1433,10 @@ void QImage::setDevicePixelRatio(qreal scaleFactor)
 {
     if (!d)
         return;
+
+    if (scaleFactor == d->devicePixelRatio)
+        return;
+
     detach();
     d->devicePixelRatio = scaleFactor;
 }

src/gui/image/qpixmap.cpp

@@ -682,6 +682,9 @@ void QPixmap::setDevicePixelRatio(qreal scaleFactor)
     if (isNull())
         return;
 
+    if (scaleFactor == data->devicePixelRatio())
+        return;
+
     detach();
     data->setDevicePixelRatio(scaleFactor);
 }