Fix EditorPropertyEasing capturing drag events originated outside of it

(cherry picked from commit 31db95b048813b78cd8259cd41b5b98db9e8e540)

editor/editor_properties.cpp

@@ -945,7 +945,7 @@ void EditorPropertyEasing::_drag_easing(const Ref<InputEvent> &p_ev) {
 
 	const Ref<InputEventMouseMotion> mm = p_ev;
 
-	if (mm.is_valid() && mm->get_button_mask() & BUTTON_MASK_LEFT) {
+	if (dragging && mm.is_valid() && mm->get_button_mask() & BUTTON_MASK_LEFT) {
 		float rel = mm->get_relative().x;
 		if (rel == 0) {
 			return;