implement input and rework window implementation

moved the window variable as a constant in the window module, and
implement more of GLFW logic, including some basic input handling.

src/io/window.h

@@ -2,11 +2,5 @@
 // Licensed under the MIT Licence. See LICENSE for details
 #pragma once
 
-#include <GLFW/glfw3.h>
-
-struct renderdat {
-	GLFWwindow* win;
-};
-
-int window_init(struct renderdat* restrict rdat);
-void window_free(struct renderdat* restrict rdat);
+int window_init(void);  // initializes the global window, returns non-zero upon failure
+void window_loop(void); // performs the window updates