Fix viewport not updating mouse pos on click.

Closes #47594. See further discussion there. Thanks to @Bhu1-V for the investigation which led to this fix.

scene/main/viewport.cpp

@@ -1830,6 +1830,7 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 		Control *over = nullptr;
 
 		Point2 mpos = mb->get_position();
+		gui.last_mouse_pos = mpos;
 		if (mb->is_pressed()) {
 			Size2 pos = mpos;
 			if (gui.mouse_focus_mask) {