Pick up changes to device-pixel-ratio in QRasterBackingStore

Regression from 2d2d9078, which assumed the code in QBackingStore::beginPaint
would apply to macOS as well, but QHighDpiScaling::isActive() is only active
when Qt does the high DPI scaling, so we never hit the resize() code-path.

Adds backingStore() accessor to go from QPlatformBackingStore to QBackingStore.

Change-Id: I5de2ade74e731df445ea22e0f49b2a121e678efe
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/gui/painting/qbackingstore.cpp

@@ -138,6 +138,7 @@ QBackingStore::QBackingStore(QWindow *window)
     : d_ptr(new QBackingStorePrivate(window))
 {
     d_ptr->platformBackingStore = QGuiApplicationPrivate::platformIntegration()->createPlatformBackingStore(window);
+    d_ptr->platformBackingStore->setBackingStore(this);
 }
 
 /*!

src/gui/painting/qplatformbackingstore.cpp

@@ -77,6 +77,7 @@ class QPlatformBackingStorePrivate
 public:
     QPlatformBackingStorePrivate(QWindow *w)
         : window(w)
+        , backingStore(0)
 #ifndef QT_NO_OPENGL
         , textureId(0)
         , blitter(0)
@@ -100,6 +101,7 @@ public:
 #endif
     }
     QWindow *window;
+    QBackingStore *backingStore;
 #ifndef QT_NO_OPENGL
     mutable GLuint textureId;
     mutable QSize textureSize;
@@ -622,6 +624,23 @@ QWindow* QPlatformBackingStore::window() const
     return d_ptr->window;
 }
 
+/*!
+    Sets the backing store associated with this surface.
+*/
+void QPlatformBackingStore::setBackingStore(QBackingStore *backingStore)
+{
+    d_ptr->backingStore = backingStore;
+}
+
+/*!
+    Returns a pointer to the backing store associated with this
+    surface.
+*/
+QBackingStore *QPlatformBackingStore::backingStore() const
+{
+    return d_ptr->backingStore;
+}
+
 /*!
     This function is called before painting onto the surface begins,
     with the \a region in which the painting will occur.

src/gui/painting/qplatformbackingstore.h

@@ -112,6 +112,7 @@ public:
     virtual ~QPlatformBackingStore();
 
     QWindow *window() const;
+    QBackingStore *backingStore() const;
 
     virtual QPaintDevice *paintDevice() = 0;
 
@@ -145,6 +146,9 @@ public:
 
 private:
     QPlatformBackingStorePrivate *d_ptr;
+
+    void setBackingStore(QBackingStore *);
+    friend class QBackingStore;
 };
 
 #ifndef QT_NO_OPENGL

src/platformsupport/graphics/qrasterbackingstore.cpp

@@ -39,6 +39,7 @@
 
 #include "qrasterbackingstore_p.h"
 
+#include <QtGui/qbackingstore.h>
 #include <QtGui/qpainter.h>
 
 QT_BEGIN_NAMESPACE
@@ -104,6 +105,10 @@ bool QRasterBackingStore::scroll(const QRegion &region, int dx, int dy)
 
 void QRasterBackingStore::beginPaint(const QRegion &region)
 {
+    // Keep backing store device pixel ratio in sync with window
+    if (m_image.devicePixelRatio() != window()->devicePixelRatio())
+        resize(backingStore()->size(), backingStore()->staticContents());
+
     if (!m_image.hasAlphaChannel())
         return;