Display statistics about the performance in interactive mode (average FPS).
Don't display individual buckets in interactive mode.

The whole frame is displayed with displayVFB() anyway; extra drawing
only slows things down.
Don't wait for ESC after rendering in interactive mode.
Optimize and debug the interactive mode a bit.

The notable difference is that mainloop() now uses renderScene() instead
of renderScene_threaded(). The latter spawns a new thread per each frame,
and uses sync primitives between the main and the rendering thread.
Removing this overhead speeds up thing significantly, reaching 35fps on
a very simple scene (compared to 9 previously).

Also, move the ThreadPool to where it belongs.