OpenGL Core Profile and an overlay with the QPainter

I've played recently with the OpenGL Core Profile and QGLWidget to see if I can render a scene with only core profile functions and shaders. Everything worked well until I decided to add an FPS counter using overlaid text drawn with the QPainter.

A little bit about Hilbert space-filling curve

As a student I have investigated properties of different scanning algorithm to unwrap an N-dimensional array into one dimensional. One of the algorithms was  Hilbert space-filling algorithm. This algorithm can be applied to an array of any dimension and it is known as one of the unwrapping algorithms that preserves spatial relations between array's neighbors very well.