Don't remove the touch grab until the last touch point is removed

Previously if you move a window around and temporarily add a second
finger then it will cancel the grab even though the original finger is
still held on the screen. It seems more robust to avoid cancelling the
grab until all fingers have been removed.

src/shell.c

@@ -1085,7 +1085,9 @@ touch_move_grab_up(struct weston_touch_grab *grab, uint32_t time, int touch_id)
 	struct shell_touch_grab *shell_grab = container_of(grab, 
 							   struct shell_touch_grab,
 							   grab);
-	shell_touch_grab_end(shell_grab);
+
+	if (grab->touch->seat->num_tp == 0)
+		shell_touch_grab_end(shell_grab);
 }
 
 static void