Made window-related callbacks per-window.

This makes polymorphic behaviour easier to implement and avoids the problem of
events being triggered before the GLFW window object is fully usable.

tests/accuracy.c

@@ -86,10 +86,6 @@ int main(void)
         exit(EXIT_FAILURE);
     }
 
-    glfwSetCursorPosCallback(cursor_position_callback);
-    glfwSetWindowSizeCallback(window_size_callback);
-    glfwSetKeyCallback(key_callback);
-
     window = glfwCreateWindow(window_width, window_height, GLFW_WINDOWED, "", NULL);
     if (!window)
     {
@@ -99,6 +95,10 @@ int main(void)
         exit(EXIT_FAILURE);
     }
 
+    glfwSetCursorPosCallback(window, cursor_position_callback);
+    glfwSetWindowSizeCallback(window, window_size_callback);
+    glfwSetKeyCallback(window, key_callback);
+
     glfwMakeContextCurrent(window);
 
     glfwGetWindowSize(window, &width, &height);