QOpenGL2PaintEngineEx: Store context via QPointer

The QOpenGLBuffers used by QOpenGL2PaintEngineEx are tied to a specific
share group on creation, via QOpenGLSharedResourceGuard.

If QOpenGL2PaintEngineEx is later used with a recreated QOpenGLContext,
that happens to live at the same address as the previous one, we will
fail to detect that the context has changed by using a naked pointer
comparison (ABA problem).

If the share group of the new context is different, this will cause
problems down the line in QOpenGLBuffers::bind(). This scenario has
been observed on Android, where the context created in
QOpenGLWidgetPrivate::initialize() ends up with the same address,
but then adopts a new share context from RHI, because the widget has
been moved to a different top level window.

To remedy this, we store the QOpenGL2PaintEngineEx's cached context
via a QPointer, so that we pick up the B state of ABA (null), and
correctly throw away the old buffers in QOpenGL2PaintEngineEx::begin.

Change-Id: I5c221a37fd95f846d96e72509bba54f3d10fee3a
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit 8f6b27fdeef4f45d70c7bc93f7393f58f5ddeb16)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/opengl/qopenglpaintengine_p.h

@@ -243,7 +243,7 @@ public:
     QOpenGLEngineShaderManager* shaderManager;
     QOpenGLPaintDevice* device;
     int width, height;
-    QOpenGLContext *ctx;
+    QPointer<QOpenGLContext> ctx;
     EngineMode mode;
     QFontEngine::GlyphFormat glyphCacheFormat;