Bug fix: use offset arg to write into the output, not to calc indices.

  function kernel(i,j) { return 1 + i*100 + j; }
  var mExpect = new Matrix([100,5], kernel);
  var mActual = new Matrix([100], [5, "any"],
      function (i, out) {
        out.gather(function (j) { return kernel(i,j); }); });

  print("1. expect: "+mExpect.get(0,0)+" actual: "+mActual.get(0,0));
Still exploring what fix is right for Cursor.gather issues.