Enablers for QQuickWidget render() support.

Add a QImage based fallback for renderToTexture widgets, and use
that when rendering to something else than a QWidgetBackingStore.

Change-Id: I415a3a27c4ecb4ddbac45181c5a568b01ac5cb7a
Task-number: QTBUG-39562
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

2 files changed, 8 insertions, 3 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 311f9547a1..a1f3fbe8db 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -5506,8 +5506,13 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 // This widget renders into a texture which is composed later. We just need to
                 // punch a hole in the backingstore, so the texture will be visible.
                 QPainter p(q);
-                p.setCompositionMode(QPainter::CompositionMode_Source);
-                p.fillRect(q->rect(), Qt::transparent);
+                if (backingStore) {
+                    p.setCompositionMode(QPainter::CompositionMode_Source);
+                    p.fillRect(q->rect(), Qt::transparent);
+                } else {
+                    // We are not drawing to a backingstore: fall back to QImage
+                    p.drawImage(q->rect(), grabFramebuffer());
+                }
             } else {
                 //actually send the paint event
                 QPaintEvent e(toBePainted);
diff --git a/src/widgets/kernel/qwidget_p.h b/src/widgets/kernel/qwidget_p.h
index 14831efca8..6520832943 100644
--- a/src/widgets/kernel/qwidget_p.h
+++ b/src/widgets/kernel/qwidget_p.h
@@ -627,7 +627,7 @@ public:
 
 #ifndef QT_NO_OPENGL
     virtual GLuint textureId() const { return 0; }
-
+    virtual QImage grabFramebuffer() const { return QImage(); }
     void setRenderToTexture() { renderToTexture = true; setTextureChildSeen(); }
     void setTextureChildSeen()
     {