QOpenGLCompositorBackingStore: ensure backing store on flushed windows

When the first QEglFSWindow got created for a QWindow, a backing store
was created with it and associated to the new QEglFSWindow. When the
window was hidden on the platform screen, the QEglFSWindow got deleted
and re-created when it was supposed to be shown.
The re-created QEglFSWindow was no longer associated to the backing
store. It was therefore not rendered on the screen.

=> ensure that the backing store is re-associated to the QEglFSWindow,
corrsponding to the QWindow argument passed to flush().

No autotest is added, because the fix is purely visual.
The widgets/menus example can be used to test the functionality
manually (see bug reports).

Fixes: QTBUG-115196
Fixes: QTBUG-116769
Pick-to: 6.6 6.5 6.2
Change-Id: I92ce2e8f7e76bd2d26e713a4d20612d079fb4717
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

Author: ASpoerl

src/opengl/qopenglcompositor_p.h

@@ -29,6 +29,7 @@ class QOpenGLFramebufferObject;
 class QWindow;
 class QPlatformTextureList;
 
+class QOpenGLCompositorBackingStore;
 class QOpenGLCompositorWindow
 {
 public:
@@ -37,6 +38,8 @@ class QOpenGLCompositorWindow
     virtual const QPlatformTextureList *textures() const = 0;
     virtual void beginCompositing() { }
     virtual void endCompositing() { }
+    virtual void setBackingStore(QOpenGLCompositorBackingStore *backingStore) = 0;
+    virtual QOpenGLCompositorBackingStore *backingStore() const = 0;
 };
 
 class Q_OPENGL_EXPORT QOpenGLCompositor : public QObject

src/opengl/qopenglcompositorbackingstore.cpp

@@ -137,6 +137,9 @@ void QOpenGLCompositorBackingStore::updateTexture()
 void QOpenGLCompositorBackingStore::flush(QWindow *window, const QRegion &region, const QPoint &offset)
 {
     // Called for ordinary raster windows.
+    auto *handle = dynamic_cast<QOpenGLCompositorWindow *>(window->handle());
+    if (handle && !handle->backingStore())
+        handle->setBackingStore(this);
 
     Q_UNUSED(region);
     Q_UNUSED(offset);

src/plugins/platforms/eglfs/api/qeglfswindow_p.h

@@ -70,8 +70,8 @@ class Q_EGLFS_EXPORT QEglFSWindow : public QPlatformWindow
     bool isRaster() const;
 
 #ifndef QT_NO_OPENGL
-    QOpenGLCompositorBackingStore *backingStore() { return m_backingStore; }
-    void setBackingStore(QOpenGLCompositorBackingStore *backingStore);
+    QOpenGLCompositorBackingStore *backingStore() const override { return m_backingStore; }
+    void setBackingStore(QOpenGLCompositorBackingStore *backingStore) override;
     QWindow *sourceWindow() const override;
     const QPlatformTextureList *textures() const override;
     void endCompositing() override;