Fix C# API assembly build errors in generics

modules/mono/glue/Managed/Files/Extensions/NodeExtensions.cs

@@ -4,7 +4,7 @@ namespace Godot
     {
         public T GetNode<T>(NodePath path) where T : class
         {
-            return (T)GetNode(path);
+            return (T)(object)GetNode(path);
         }
 
         public T GetNodeOrNull<T>(NodePath path) where T : class
@@ -14,7 +14,7 @@ namespace Godot
 
         public T GetChild<T>(int idx) where T : class
         {
-            return (T)GetChild(idx);
+            return (T)(object)GetChild(idx);
         }
 
         public T GetChildOrNull<T>(int idx) where T : class
@@ -24,7 +24,7 @@ namespace Godot
 
         public T GetOwner<T>() where T : class
         {
-            return (T)GetOwner();
+            return (T)(object)GetOwner();
         }
 
         public T GetOwnerOrNull<T>() where T : class
@@ -34,7 +34,7 @@ namespace Godot
 
         public T GetParent<T>() where T : class
         {
-            return (T)GetParent();
+            return (T)(object)GetParent();
         }
 
         public T GetParentOrNull<T>() where T : class

modules/mono/glue/Managed/Files/Extensions/ResourceLoaderExtensions.cs

@@ -2,9 +2,9 @@ namespace Godot
 {
     public static partial class ResourceLoader
     {
-        public static T Load<T>(string path) where T : Godot.Resource
+        public static T Load<T>(string path) where T : class
         {
-            return (T) Load(path);
+            return (T)(object)Load(path);
         }
     }
 }

modules/mono/glue/Managed/Files/GD.cs

@@ -67,7 +67,7 @@ namespace Godot
 
         public static T Load<T>(string path) where T : class
         {
-            return (T) ResourceLoader.Load(path);
+            return ResourceLoader.Load<T>(path);
         }
 
         public static void Print(params object[] what)