Add a starfield in the background.

This should help to better visualize the velocity of the ship.

src/starfield.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2026  Ortega Froysa, Nicolás <nicolas@ortegas.org>
+ * Author: Ortega Froysa, Nicolás <nicolas@ortegas.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+/**
+ * @brief Draw stars in the visible area of the starfield.
+ *
+ * @param camera_x The x position of the camera center.
+ * @param camera_y The y position of the camera center.
+ * @param zoom The zoom level (scale factor).
+ */
+void starfield_draw(float camera_x, float camera_y, float zoom);