Pan laws and stereo placement

When a mono source moves between speakers, its perceived loudness changes unless the gains follow a pan law. ferraus uses constant power by default.

`--law`	Center gain per side	Notes
`-3dB` (default)	-3 dB	constant power: summed power is the same at every position
`-4.5dB`	-4.5 dB	compromise used by some consoles
`-6dB`	-6 dB	constant amplitude: mono-compatible sum at the center
`linear`	-6 dB, linear ramp	simple crossfade

Positions run from -100 (hard left) through 0 (center) to +100 (hard right); left, center, right, L30, R60 also work.

pan --mode pan places a mono (or mono-summed) source. --mode balance scales the channels of a stereo source instead, keeping its image.
Movement: pan --points 0s=-100,4s=0,8s=100 --curve spline or --from -100 --to 100 --over 0s..end.
Cyclic movement: autopan --rate 1/2 (beats) or --rate 0.5Hz.
While mixing: mix vox.wav gtr.wav keys.wav --gains 0dB,-3dB,-6dB --pans 0,-40,+40.
widen --amount 130% --mono-below 120Hz widens the image but keeps the bass mono.