Determine which alignment segment to render as an arrow. This is either the first or last 'M' section, excluding soft clipping.
function getArrowIndex(read: Alignment): number {
  var i, op, ops = read.cigarOps;
  if (read.getStrand() == '-') {
    for (i = 0; i < ops.length; i++) {
      op = ops[i];
      if (op.op == 'S') continue;
      if (op.op == 'M') return i;
      return -1;
    }
  } else {
    for (i = ops.length - 1; i >= 0; i--) {
      op = ops[i];
      if (op.op == 'S') continue;
      if (op.op == 'M') return i;
      return -1;
    }
  }
  return -1;
}