Add a panning filter.

Add a panning filter.

This does a simple left/right balance when channel=-1 (default). When
channel >= 0, you can adjust an individual channel's left/right
position. Whereas the simple balance will not cause one channel to
appear in another channel, the individual channel does. The start/end
properties are floats in the range [-1.0, 1.0]. A start property alone
makes it constant over the duration of the filter. There is some
handling for more than 2 channels by providing front/rear fade and
ganging (balance front and rear together or fade left and right
together).