Notify child controls when BackBufferCopy's rect changed

(cherry picked from commit 602a0d2fbc84d598cffc79eccd5110f650b6e3c9)

scene/2d/back_buffer_copy.cpp

@@ -62,6 +62,7 @@ Rect2 BackBufferCopy::get_anchorable_rect() const {
 void BackBufferCopy::set_rect(const Rect2 &p_rect) {
 	rect = p_rect;
 	_update_copy_mode();
+	item_rect_changed();
 }
 
 Rect2 BackBufferCopy::get_rect() const {