Add generic support to PackedScene.Instance

(cherry picked from commit 3f50954ced)

modules/mono/glue/GodotSharp/GodotSharp/Core/Extensions/PackedSceneExtensions.cs

@@ -0,0 +1,27 @@
+namespace Godot
+{
+    public partial class PackedScene
+    {
+        /// <summary>
+        /// Instantiates the scene's node hierarchy, erroring on failure.
+        /// Triggers child scene instantiation(s). Triggers a
+        /// `Node.NotificationInstanced` notification on the root node.
+        /// </summary>
+        /// <typeparam name="T">The type to cast to. Should be a descendant of Node.</typeparam>
+        public T Instance<T>(PackedScene.GenEditState editState = (PackedScene.GenEditState)0) where T : class
+        {
+            return (T)(object)Instance(editState);
+        }
+
+        /// <summary>
+        /// Instantiates the scene's node hierarchy, returning null on failure.
+        /// Triggers child scene instantiation(s). Triggers a
+        /// `Node.NotificationInstanced` notification on the root node.
+        /// </summary>
+        /// <typeparam name="T">The type to cast to. Should be a descendant of Node.</typeparam>
+        public T InstanceOrNull<T>(PackedScene.GenEditState editState = (PackedScene.GenEditState)0) where T : class
+        {
+            return Instance(editState) as T;
+        }
+    }
+}

modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj

@@ -28,6 +28,7 @@
     <Compile Include="Core\DynamicObject.cs" />
     <Compile Include="Core\Extensions\NodeExtensions.cs" />
     <Compile Include="Core\Extensions\ObjectExtensions.cs" />
+    <Compile Include="Core\Extensions\PackedSceneExtensions.cs" />
     <Compile Include="Core\Extensions\ResourceLoaderExtensions.cs" />
     <Compile Include="Core\GD.cs" />
     <Compile Include="Core\GodotSynchronizationContext.cs" />