Spectrum Analyser draws the wrong number of bars

The `analyser.getByteFrequencyData` method gives us an array of values, which is just what we want! Unfortunately, the number of values in the array is always a power of two (half of `analyser.fftSize`) and we need 19.

I think our only option is to use far more bins than we need, and simply average length/19 of the bins for each bar. It's going to lead to some inaccuracy along the way, but I can't see a better approach.

Currently we are getting 32 bins and drawing all of them, even though many are off the canvas. Gross.