spectrogram: fix and improve the visualizer
Following the compiler warning about a Xor-used-as-a-pow, I've checked if the visualizer was running normally, he was not :
The top screenshots are from Audacity and match the theory.
On the other hand, the two next screenshots are from the original VLC visualizer which are clearly irrelevant.
By fixing the issue, we get the following result :
We can consider that these results match with Audacity and are correct, despite the weird-looking display of the high frequency of the squared audio. This is due to the high peak frequencies which are closer and closer and finally merge sometimes.