SplineSource() supports spline interpolating multi-channel tabular data,
cropping it and generating wavesynth compatible segment data from it.
ComposingSplineSource() needs some verification still.
* "trigger" now means that the corresponding line will only start
once the trigger line is high.
* "jump" is implicit as the last line in a frame must jump back.
* spline coefficients are now compensated for finite time step size
