Merge pull request #1390 from brollb/1207-go-back-layers

'go to back' on large num in 'go back' block. Fixes #1207

objects.js

@@ -3201,12 +3201,15 @@ SpriteMorph.prototype.comeToFront = function () {
 };
 
 SpriteMorph.prototype.goBack = function (layers) {
-    var layer, newLayer = +layers || 0;
+    var layer,
+        newLayer = +layers,
+        targetLayer;
+
     if (!this.parent) {return null; }
     layer = this.parent.children.indexOf(this);
-    if (layer < newLayer) {return null; }
     this.parent.removeChild(this);
-    this.parent.children.splice(layer - newLayer, null, this);
+    targetLayer = Math.max(layer - newLayer, 0);
+    this.parent.children.splice(targetLayer, null, this);
     this.parent.changed();
 };