Adding glow pulse visualization

[phayes]

Please find first-cut of glow pulse visualization on the album art on the player.

Changes to existing code:

1. Spectrum now reads from before volume instead of after, so volume changes don't affect visualization.
2. Spectrum was improved to add a few more common things needed by visualizers instead of just frequency bands. I added lows, mids, and highs, as well as peak and rms.

New code:

Most of the new code is in frame_glow.rs . I think it's a bit overly complex for now for what it does, but I want to get feedback on the visual effect before stripping it down to basics (I originally tried for a more complex visualization than I ended up using, and much of that original machinery is still in place).

What I would like feedback on:

1. Overall impressions of the glow
2. Is the glow too-big or too bright?
3. How's the colour? Do we want to colour-match it to the album better?

Once I get this feedback I'll tweak it and simplify the code a bit.