Support separate roughing and finishing

[em]

Roughing buffers variable stresses that affect finish by first doing a rough cut and then following up with a finishing pass under nearly ideal circumstances and thus better results all around.

Note: Ideal circumstances has arguably more to do with consistent stress than less stress. 1 depth of cut should be ideal and we already have the number.

Not sure how to do the API. Maybe all that's really needed is a context.globalOffset so it can be done with setup blocks:

setup('1/2" roughing', function(ctx) {
  ctx.globalOffset = -ctx.depthOfCut;
  whatever();
});

setup('1/8" finishing', function(ctx) {
   whatever();
});

Gonna have to wait until strokes use the filling algorithm for this to work.