QNX: Fix OpenGL autotest

On QNX grabbing the frame buffer returns the content of the back buffer.
In order to execute the OpenGL tests properly a swapBuffers is executed before
in order to be able to retrieve the content of the front buffer.

The patch also documents this platform behavior.

Change-Id: I7a501818ec6eea061f2f54f95854f68cb72e0534
Reviewed-by: James McDonnell <jmcdonnell@qnx.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/opengl/qgl.cpp

@@ -4083,6 +4083,11 @@ QPixmap QGLWidget::renderPixmap(int w, int h, bool useContext)
     Depending on your hardware, you can explicitly select which color
     buffer to grab with a glReadBuffer() call before calling this
     function.
+
+    On QNX the back buffer is not preserved when swapBuffers() is called. The back buffer
+    where this function reads from, might thus not contain the same content as the front buffer.
+    In order to retrieve what is currently visible on the screen, swapBuffers()
+    has to be executed prior to this function call.
 */
 QImage QGLWidget::grabFrameBuffer(bool withAlpha)
 {

src/opengl/qglframebufferobject.cpp

@@ -1114,6 +1114,9 @@ QGLFramebufferObjectFormat QGLFramebufferObject::format() const
     \fn QImage QGLFramebufferObject::toImage() const
 
     Returns the contents of this framebuffer object as a QImage.
+
+    On QNX the back buffer is not preserved when a buffer swap occures. So this function
+    might return old content.
 */
 QImage QGLFramebufferObject::toImage() const
 {

tests/auto/opengl/qgl/tst_qgl.cpp

@@ -1064,6 +1064,12 @@ void tst_QGL::glWidgetRendering()
     QVERIFY(w.beginOk);
     QVERIFY(w.engineType == QPaintEngine::OpenGL || w.engineType == QPaintEngine::OpenGL2);
 
+#if defined(Q_OS_QNX)
+    // glReadPixels reads from the back buffer. On QNX the buffer is not preserved
+    // after a buffer swap. This is why we have to swap the buffer explicitly before calling
+    // grabFrameBuffer to retrieve the content of the front buffer.
+    w.swapBuffers();
+#endif
     QImage fb = w.grabFrameBuffer(false);
     qt_opengl_check_test_pattern(fb);
 }
@@ -1104,6 +1110,9 @@ void tst_QGL::glFBOSimpleRendering()
 //       buffer is actually missing. But that's probably ok anyway.
 void tst_QGL::glFBORendering()
 {
+#if defined(Q_OS_QNX)
+    QSKIP("Reading the QGLFramebufferObject is unsupported on this platform");
+#endif
     if (!QGLFramebufferObject::hasOpenGLFramebufferObjects())
         QSKIP("QGLFramebufferObject not supported on this platform");
 
@@ -1280,7 +1289,7 @@ protected:
         fboPainter.end();
         fboImage = fbo->toImage();
 
-        widgetPainter.fillRect(-1, -1, width()+2, width()+2, Qt::blue);
+        widgetPainter.fillRect(-1, -1, width()+2, height()+2, Qt::blue);
 
         delete fbo;
     }
@@ -1301,6 +1310,13 @@ void tst_QGL::glFBOUseInGLWidget()
     QVERIFY(w.widgetPainterBeginOk);
     QVERIFY(w.fboPainterBeginOk);
 
+#if defined(Q_OS_QNX)
+    // glReadPixels reads from the back buffer. On QNX the buffer is not preserved
+    // after a buffer swap. This is why we have to swap the buffer explicitly before calling
+    // grabFrameBuffer to retrieve the content of the front buffer
+    w.swapBuffers();
+#endif
+
     QImage widgetFB = w.grabFrameBuffer(false);
     QImage widgetReference(widgetFB.size(), widgetFB.format());
     widgetReference.fill(0xff0000ff);
@@ -1691,6 +1707,12 @@ void tst_QGL::replaceClipping()
     glw.paint(&referencePainter);
     referencePainter.end();
 
+#if defined(Q_OS_QNX)
+    // glReadPixels reads from the back buffer. On QNX the buffer is not preserved
+    // after a buffer swap. This is why we have to swap the buffer explicitly before calling
+    // grabFrameBuffer to retrieve the content of the front buffer
+    glw.swapBuffers();
+#endif
     const QImage widgetFB = glw.grabFrameBuffer(false).convertToFormat(QImage::Format_RGB32);
 
     // Sample pixels in a grid pattern which avoids false failures due to
@@ -1802,6 +1824,12 @@ void tst_QGL::clipTest()
     glw.paint(&referencePainter);
     referencePainter.end();
 
+#if defined(Q_OS_QNX)
+    // glReadPixels reads from the back buffer. On QNX the buffer is not preserved
+    // after a buffer swap. This is why we have to swap the buffer explicitly before calling
+    // grabFrameBuffer to retrieve the content of the front buffer
+    glw.swapBuffers();
+#endif
     const QImage widgetFB = glw.grabFrameBuffer(false).convertToFormat(QImage::Format_RGB32);
 
     // Sample pixels in a grid pattern which avoids false failures due to