Sets the root save directory for saving screenshots.
    
    Screenshots will be saved in subdirectories under this directory by
    browser window size.

def set_save_directory(base, source):
    """Sets the root save directory for saving screenshots.
    
    Screenshots will be saved in subdirectories under this directory by
    browser window size. """
    root = os.path.join(base, source)
    if not os.path.isdir(root):
        os.makedirs(root)

    world.screenshot_root = root