Revise audio filtering: Use window functions

[danfrz]

Use a window function before filtering, that should make filtering actually sound reasonable I hope.

Also, this turns out to be the proper solution to the problem, not adding more song data to the buffer of the filter operating. As a side note, keep the buffer larger than input data and power of 2n, but instead of copying song data in there, have zeroes. Apparently that can make filtering sound better.

This makes the fourier_buffer_size byte within songs unnecessary.
Also, may end up making window_type option for tracks, set by a function within the filter table.