Image derivative generation does not work if effect "Convert" in use and file stored in private filesystem

Problem/Motivation

If file is converted to another format, i.e. from jpg to gif, filepath does not match anymore. We already have a piece of code to match converted image uri but it works only for non-private files.

See Drupal\image\Controller\ImageStyleDownloadController::deliver():

    // Don't try to generate file if source is missing.
    if (!file_exists($image_uri)) {
      // If the image style converted the extension, it has been added to the
      // original file, resulting in filenames like image.png.jpeg. So to find
      // the actual source image, we remove the extension and check if that
      // image exists.
      $path_info = pathinfo($image_uri);
      $converted_image_uri = $path_info['dirname'] . DIRECTORY_SEPARATOR . $path_info['filename'];
      if (!file_exists($converted_image_uri)) {
        $this->logger->notice('Source image at %source_image_path not found while trying to generate derivative image at %derivative_path.', array('%source_image_path' => $image_uri, '%derivative_path' => $derivative_uri));
        return new Response($this->t('Error generating image, missing source file.'), 404);
      }
      else {
        // The converted file does exist, use it as the source.
        $image_uri = $converted_image_uri;
      }
    }

Proposed resolution

1. Wait for #2702227: Image styles for private files are serving the original instead of derivative
2. Change code order - first use correct $image_uri ($converted_image_uri), than check if user has access to original file.

Remaining tasks

as above

User interface changes

none

API changes

none

Data model changes

none