pixeltool: Pick color under cursor also outside of the pixeltool window

We want to pick color directly from the whole screen

Change-Id: Ifc845b1303203db54c657dac6fa02ba841b97fcc
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/pixeltool/qpixeltool.cpp

@@ -553,6 +553,7 @@ void QPixelTool::grabScreen()
 
     update();
 
+    m_currentColor = m_buffer.toImage().pixel(m_buffer.rect().center());
     m_lastMousePos = mousePos;
 }