QGLFramebufferObject's QGLPaintDevice is initialized with wrong GL format

If the QGLFramebufferObject internal texture format is set as GL_RGB8,
QGLPaintDevice::alphaRequested() should return false, but it always returns true.

The QGLFBOGLPaintDevice::setFBO() is called too early in QGLFramebufferObjectPrivate::init,
it needs to be called last in init, after the FBO format is correctly set.

The reason this is important is because the GL fontengine will switch to RASTER_A8 font rendering instead of RGBMask ClearType rendering,
when a QPainter is attached to a QGLFramebufferObject with internal format GL_RGB8, which results in a different font rendering than when using QGLWidget as the paint device.

I attached a patch that fixes the problem.