Updated our shaders to use Z to determine color.

This commit updates our vertex and fragment shaders to communicate via a
varying variable. The fragment shader uses this varying variable - which
is the Z component of each vertex - to determine the color of the
vertex.
Implemented the computation of STFT magnitude values.

This commit changes the way our spectrogram vertices are computed. We
take the magnitude value from each DFT result, take the base-10
logarithm of it, and then scale it to a range relative to the minimum
and maximum magnitude in the input file.