Viewer: add SpotLightHelper to view spotlight's cone

+ add function set_folder_callback

src/index.js

@@ -1143,12 +1143,24 @@ class Viewer {
         return spot_light;
     }
 
+    set_folder_callback(path, callback) {
+        this.scene_tree.find(path).on_update = callback;
+    }
+
     add_default_scene_elements() {
         var spot_light = this.create_default_spot_light();
         this.set_object(["Lights", "SpotLight"], spot_light);
         // By default, the spot light is turned off, since
         // it's primarily used for casting detailed shadows
         this.set_property(["Lights", "SpotLight"], "visible", false);
+        const spotlight_helper = new THREE.SpotLightHelper(spot_light, 0xf5a716);
+        spotlight_helper.matrix = spot_light.matrix;
+        this.set_folder_callback(["Lights", "SpotLight"], () => {
+            spotlight_helper.update();
+            this.set_dirty();
+        });
+        this.set_object(["Lights", "SpotLightHelper"], spotlight_helper);
+        this.set_property(["Lights", "SpotLightHelper"], "visible", false);
 
         var point_light_px = new THREE.PointLight(0xffffff,
                                                   this.upgrade_intensity(0.4));
@@ -1226,9 +1238,9 @@ class Viewer {
         bg_folder.on_update = () => {
             this.update_background();
         };
-        this.scene_tree.find(["Background", "<object>"]).on_update = () => {
+        this.set_folder_callback(["Background", "<object>"], () => {
             this.update_background();
-        }
+        })
         this.update_background();
     }
 

yarn.lock

@@ -1111,13 +1111,20 @@ __metadata:
   languageName: node
   linkType: hard
 
-"three@npm:^0.160.1, three@npm:~0.160.0":
+"three@npm:^0.160.1":
   version: 0.160.1
   resolution: "three@npm:0.160.1"
   checksum: b14b54a8b0799bc337ea2a841a1f01322dc50198d8a438ff4fbdf83ad95a15bd405ba43bd398c05b1ffc2e40a7685b73481a048c288389acb949cea5510269ea
   languageName: node
   linkType: hard
 
+"three@npm:~0.160.0":
+  version: 0.160.0
+  resolution: "three@npm:0.160.0"
+  checksum: d78e9605de6f8072e7fbebe36db7741c13f2ae89be15b4421d2fc7d549e216f53cb3e136d98e9424648f19a42345f47f8186478e038f0046655475166c156b0b
+  languageName: node
+  linkType: hard
+
 "update-browserslist-db@npm:^1.0.13":
   version: 1.0.13
   resolution: "update-browserslist-db@npm:1.0.13"