handle window exits more gracefully and appropriately.

src/io/window.h

@@ -2,5 +2,18 @@
  * Licensed under the MIT Licence. See LICENSE for details */
 #pragma once
 
-int window_init(void);  // initializes the global window, returns non-zero upon failure
-void window_loop(void); // performs the window updates
+/* Set up the window, enabling OpenGL, and
+ * configuring the settings that are needed.
+ * Returns `0` upon success, otherwise `1`. */
+int window_init(void);
+
+/* Calls the update loop for the window.
+ * This function does not exit until the window does. */
+void window_loop(void);
+
+/* Requests the window to close (gracefully). */
+void window_close(void);
+
+/* Cleans up all resources held by the window.
+ * If the window is still open, it will be terminated. */
+void window_free(void);