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.

examples/boing.c

@@ -577,8 +577,6 @@ int main( void )
       exit( EXIT_FAILURE );
    }
 
-   glfwSetWindowSizeCallback( reshape );
-
    glfwWindowHint(GLFW_DEPTH_BITS, 16);
 
    window = glfwCreateWindow( 400, 400, GLFW_WINDOWED, "Boing (classic Amiga demo)", NULL );
@@ -589,6 +587,8 @@ int main( void )
        exit( EXIT_FAILURE );
    }
 
+   glfwSetWindowSizeCallback(window, reshape);
+
    glfwMakeContextCurrent(window);
    glfwSwapInterval( 1 );