QGL2PaintEngineEx::begin does not cache QGLExtensionMatcher

In QGL2PaintEngineEx::begin (src\opengl\gl2paintengineex\qpaintengineex_opengl2.cpp line 2099 and following) a QGLExtensionMatcher is created each time. This in turn causes a call to glGetString(GL_EXTENSIONS) which is relatively computationally expensive, because it returns the string of all GL extension enabled for the render context.

#if !defined(QT_OPENGL_ES_2)
    QGLExtensionMatcher extensions;
    d->hasCompatibilityExtension = extensions.match("GL_ARB_compatibility");

    bool success = qt_resolve_version_2_0_functions(d->ctx)
                   && qt_resolve_buffer_extensions(d->ctx)
                   && (!QGLFramebufferObject::hasOpenGLFramebufferObjects()
                       || qt_resolve_framebufferobject_extensions(d->ctx));
    Q_ASSERT(success);
    Q_UNUSED(success);
#endif

In QT 5, the same is treated by a call to the context

d->initializeOpenGLFunctions();

This bug was introduced by fix for QTBUG-28284, see:

https://github.com/qt/qt/commit/11aed7c927d7ad8130d2b18a110a3f557c41e9a1

I would suggest a revert, because the QTBUG-28284 involves only rare cases, rather than affecting the performance of all QT 4.8 OpenGL programs.

A proper fix would move the boolean hasCompatibilityExtension into the QGLContext and initialize it with the other extensions.