Save a detach if QBrush::setColor does not change the brush color

We can save detaching a QBrush when setColor is called trying to set
the current color.

Change-Id: I8f4042325d0f9c2bda69d469d6861e3cc310f329
Reviewed-by: Samuel Rødal <srodal@gmail.com>

src/gui/painting/qbrush.cpp

@@ -708,6 +708,9 @@ void QBrush::setStyle(Qt::BrushStyle style)
 
 void QBrush::setColor(const QColor &c)
 {
+    if (d->color == c)
+        return;
+
     detach(d->style);
     d->color = c;
 }