Make `Gradient` resort points on `reverse`

(cherry picked from commit 3795677f3d85d0d5a741b9b040a423d8f3ad3569)

scene/resources/gradient.cpp

@@ -153,6 +153,7 @@ void Gradient::reverse() {
 		points.write[i].offset = 1.0 - points[i].offset;
 	}
 
+	is_sorted = false;
 	_update_sorting();
 	emit_signal(CoreStringNames::get_singleton()->changed);
 }