Pad Mel in the frequency dimension 1/2 gabor filter height top and 1/2 bottom

[rohrlich]

The time dimension already has an optional border of actual signal (i.e. not padding). Maybe set the default border size based on the filter width!

[rcoreilly]

Is there enough "superfluous" signal at the low end to serve as padding? maybe add more finer-grained mel channels below meaningful freq?