Input: add getSatoshisBuffer method

lib/transaction/input/input.js

@@ -154,6 +154,13 @@ Input.prototype.getSignatures = function() {
   );
 };
 
+Input.prototype.getSatoshisBuffer = function () {
+  $.checkState(this.output instanceof Output);
+  $.checkState(this.output._satoshisBN);
+  return new BufferWriter().writeUInt64LEBN(this.output._satoshisBN).toBuffer();
+};
+
+
 Input.prototype.isFullySigned = function() {
   throw new errors.AbstractMethodInvoked('Input#isFullySigned');
 };