fix(Examples): ensure constructor scene is available in build settings

Previously, the constructor scene was removed from being injected into
the example scenes as it wasn't working in a build, instead the
constructor scene was required to be the last scene in the build
settings and was loaded in by index.

This only worked if using the project settings from the VRTK repo but
would not work if VRTK was imported into another project.

To remedy this, the scene injection is now being used again and the
last scene index is being used as a fallback. If no scene is in the
build settings then the script will attempt to add it as the last
scene automatically.

Assets/VRTK/Examples/ExampleResources/SharedResources/Scripts/VRTKExample_AdditiveSceneLoader.cs

@@ -1,10 +1,16 @@
 namespace VRTK.Examples.Utilities
 {
+#if UNITY_EDITOR
+    using UnityEditor;
+#endif
     using UnityEngine;
     using UnityEngine.SceneManagement;
 
+    [ExecuteInEditMode]
     public class VRTKExample_AdditiveSceneLoader : MonoBehaviour
     {
+        [Tooltip("The constructor scene containing the VRTK SDK Manager setup to load into the scene.")]
+        public Object sceneConstructor;
         [Tooltip("The GameObject to inject into the VRTK SDK Manager as the Left Controller Script Alias.")]
         public GameObject leftScriptAlias;
         [Tooltip("The GameObject to inject into the VRTK SDK Manager as the Right Controller Script Alias.")]
@@ -16,13 +22,43 @@ public class VRTKExample_AdditiveSceneLoader : MonoBehaviour
 
         protected VRTK_SDKSetupSwitcher setupSwitcher;
         protected int constructorSceneIndex;
+        protected string constructorPath = "Assets/VRTK/Examples/VRTK_SDKManager_Constructor.unity";
 
         protected virtual void Awake()
         {
-            constructorSceneIndex = SceneManager.sceneCountInBuildSettings - 1;
-            ToggleScriptAlias(false);
-            SceneManager.sceneLoaded += OnSceneLoaded;
-            SceneManager.LoadScene(constructorSceneIndex, LoadSceneMode.Additive);
+            if (!Application.isPlaying && Application.isEditor)
+            {
+                ManageBuildSettings();
+            }
+            else
+            {
+                constructorSceneIndex = SceneManager.sceneCountInBuildSettings - 1;
+                ToggleScriptAlias(false);
+                SceneManager.sceneLoaded += OnSceneLoaded;
+                if (sceneConstructor != null)
+                {
+                    SceneManager.LoadScene(sceneConstructor.name, LoadSceneMode.Additive);
+                }
+                else
+                {
+                    SceneManager.LoadScene(constructorSceneIndex, LoadSceneMode.Additive);
+                }
+            }
+        }
+
+        protected virtual void ManageBuildSettings()
+        {
+#if UNITY_EDITOR
+            EditorBuildSettingsScene[] currentBuildScenes = EditorBuildSettings.scenes;
+            if (currentBuildScenes.Length == 0 || currentBuildScenes[currentBuildScenes.Length - 1].path != constructorPath)
+            {
+                EditorBuildSettingsScene[] newBuildScenes = new EditorBuildSettingsScene[currentBuildScenes.Length + 1];
+                System.Array.Copy(currentBuildScenes, newBuildScenes, currentBuildScenes.Length);
+                EditorBuildSettingsScene sceneToAdd = new EditorBuildSettingsScene(constructorPath, true);
+                newBuildScenes[newBuildScenes.Length - 1] = sceneToAdd;
+                EditorBuildSettings.scenes = newBuildScenes;
+            }
+#endif
         }
 
         protected virtual void LateUpdate()
@@ -35,7 +71,7 @@ protected virtual void LateUpdate()
 
         protected virtual void OnSceneLoaded(Scene loadedScene, LoadSceneMode loadMode)
         {
-            if (loadedScene.buildIndex == constructorSceneIndex)
+            if (IsConstructorScene(loadedScene))
             {
                 VRTK_SDKManager sdkManager = FindObjectOfType<VRTK_SDKManager>();
                 sdkManager.gameObject.SetActive(false);
@@ -54,6 +90,11 @@ protected virtual void OnSceneLoaded(Scene loadedScene, LoadSceneMode loadMode)
             }
         }
 
+        protected virtual bool IsConstructorScene(Scene checkScene)
+        {
+            return (checkScene != null && ((sceneConstructor != null && checkScene.name == sceneConstructor.name) || (sceneConstructor == null && checkScene.buildIndex == constructorSceneIndex)));
+        }
+
         protected virtual void ToggleScriptAlias(bool state)
         {
             if (leftScriptAlias != null)