Use struct instead of class for core structures

include/godot_cpp/variant/rect2.hpp

@@ -37,16 +37,11 @@
 
 namespace godot {
 
-class Rect2i;
 class String;
-class Transform2D;
+struct Rect2i;
+struct Transform2D;
 
-class Rect2 {
-	_FORCE_INLINE_ GDNativeTypePtr _native_ptr() const { return (void *)this; }
-
-	friend class Variant;
-
-public:
+struct _NO_DISCARD_ Rect2 {
 	Point2 position;
 	Size2 size;