Flexible scheme and URI for image derivatives

Now modules are able to alter the image style derivative generated URI, by implementing the new hook hook_image_style_uri_alter().

In the next example, the mymodule module, implements this hook to change the scheme of a derivative from private:// to public://:

function mymodule_image_style_uri_alter() {
  $name = $context['image_style']->id();
  if ($name === 'watermarked_low_resolution') {
    $path = $context['path'];
    // Low resolution, watermarked images are available publicly, in the
    // public:// stream wrapper.
    $uri = "public://styles/$name/public/$path";
  }
}