[spectrum] Avoid zero height bars that cause display artefacts

The spectrum visualization displays glitchy colours when audio is quiet and bar heights are zero.
This is because two different coloured triangles are drawn at exactly the same coordinates.

See:
https://dl.dropboxusercontent.com/u/3669512/temp/screenshot005.png

Don't allow a zero height bar and it looks like this:
https://dl.dropboxusercontent.com/u/3669512/temp/screenshot006.png