Use forward declares for vector math types

Adds operators to convert from int vector types to float vector types
as done in the upstream engine implementations.

include/godot_cpp/variant/rect2i.hpp

@@ -31,11 +31,15 @@
 #ifndef GODOT_RECT2I_HPP
 #define GODOT_RECT2I_HPP
 
-#include <godot_cpp/variant/rect2.hpp>
+#include <godot_cpp/classes/global_constants.hpp>
+#include <godot_cpp/core/math.hpp>
 #include <godot_cpp/variant/vector2i.hpp>
 
 namespace godot {
 
+class Rect2;
+class String;
+
 class Rect2i {
 	_FORCE_INLINE_ GDNativeTypePtr _native_ptr() const { return (void *)this; }
 
@@ -206,15 +210,10 @@ public:
 		return position + size;
 	}
 
-	operator String() const { return String(position) + ", " + String(size); }
-
-	operator Rect2() const { return Rect2(position, size); }
+	operator String() const;
+	operator Rect2() const;
 
 	Rect2i() {}
-	Rect2i(const Rect2 &p_r2) :
-			position(p_r2.position),
-			size(p_r2.size) {
-	}
 	Rect2i(int p_x, int p_y, int p_width, int p_height) :
 			position(Point2i(p_x, p_y)),
 			size(Size2i(p_width, p_height)) {