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/fsaa.c

@@ -93,9 +93,6 @@ int main(int argc, char** argv)
     else
         printf("Requesting that FSAA not be available\n");
 
-    glfwSetKeyCallback(key_callback);
-    glfwSetWindowSizeCallback(window_size_callback);
-
     glfwWindowHint(GLFW_FSAA_SAMPLES, samples);
 
     window = glfwCreateWindow(800, 400, GLFW_WINDOWED, "Aliasing Detector", NULL);
@@ -107,6 +104,9 @@ int main(int argc, char** argv)
         exit(EXIT_FAILURE);
     }
 
+    glfwSetKeyCallback(window, key_callback);
+    glfwSetWindowSizeCallback(window, window_size_callback);
+
     glfwMakeContextCurrent(window);
     glfwSwapInterval(1);