I'm a bit curious about the choice of OpenGL 3.3. When OpenGL 4.0 came out in 2010 it was a game changer for me and I remember it made everything (apart from quick debug drawing) so much nicer. OpenGL 3.3 was released simultaneously and was essentially as much of 4.0 as could be backported for those unable to move over right away.
Is this choice a hint at how long you've been writing this, is it comfort and familiarity? I'm not criticising the choice, it obviously works for you. I'm just interested in the reasons.