Make QGLPaintEngine work on retina displays.

QGLWidgetGLPaintDevice::size now returns the size
in device pixels.

This fixes the QPainter "overpainting" and GraphicsView
with a QGLWidget viewport use cases.

Task-number: QTBUG-30217

Change-Id: I01998d39d7b4d755cf8b7b3fab5f75cd0d899ccf
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>

src/opengl/qglpaintdevice.cpp

@@ -44,6 +44,7 @@
 #include <private/qglpixelbuffer_p.h>
 #include <private/qglframebufferobject_p.h>
 #include <qopenglfunctions.h>
+#include <qwindow.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -188,7 +189,12 @@ void QGLWidgetGLPaintDevice::endPaint()
 
 QSize QGLWidgetGLPaintDevice::size() const
 {
+#ifdef Q_OS_MAC
+    return glWidget->size() * (glWidget->windowHandle() ?
+                               glWidget->windowHandle()->devicePixelRatio() : qApp->devicePixelRatio());
+#else
     return glWidget->size();
+#endif
 }
 
 QGLContext* QGLWidgetGLPaintDevice::context() const