QGraphicsView with OpenGL viewport calls illegal GLX pixmap functions

(Running Qt 4.6 on a Laptop with Intel Corporation Mobile 915GM/GMS/910GML Express graphics chipset. Operating system is Arch Linux, graphics driver is xf86-video-intel-2.9.1-1. Output of glxinfo says "direct rendering: Yes", "GLX version: 1.2" and "OpenGL version string: 1.4 Mesa 7.7".)

Look at the stripped-down example attached below:

Without the setCacheMode line (no pixmap drawing!!), the program runs without error messages and the rectangle appears inside the QGraphicsView.

With the setCacheMode line, the rectangle doesn't appear and instead the following is shown at the command line where the program was started:

######[commandline output]######
WARNING: Application calling GLX 1.3 function "glXCreatePixmap" when GLX 1.3 is not supported! This is an application bug!
WARNING: Application calling GLX 1.3 function "glXDestroyPixmap" when GLX 1.3 is not supported! This is an application bug!
################################

Here's the source code for the example:

######[main.cpp]################
#include <QApplication>
#include <QtGui>
#include <QGLFormat>

int main(int argc, char **argv)
{
QApplication app(argc, argv);

QGraphicsScene scene;
QGraphicsItem *item = scene.addRect(0, 0, 100, 100);
item->setCacheMode(QGraphicsItem::DeviceCoordinateCache);

QGraphicsView view(&scene);
view.setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
view.show();

return app.exec();
}
################################

######[bug.pro]#################
TEMPLATE = app
QT += opengl
SOURCES += main.cpp
################################