Accurate timers

[tlecomte]

The 2D spectrogram has issues with the painting time accuracy. This is mostly noticeable with generated signals, such as an exponential sweep.

It may be related to : http://www.qtcentre.org/threads/40317-frame-rate-and-QTimer-accuracy?s=2b3923a7c9c9fcbb01cc8eb9260c0012 http://www.qtcentre.org/threads/23167-How-can-I-get-a-30-or-60-frame-rate-when-using-QGLWidget-QTimer-is-not-acurate http://www.folding-hyperspace.com/program_tip_14.htm