Rotate effect

See also #758628: Improve GD library hook_requirements() for color and image modules

That issue provides a patch that performs a requirements check for imagerotate and imagefilter support and displays a warning if they are not available.

However, we might want to also consider implementing something like #4 here where we hide the desaturate and rotate effects if they are not available. I'm not sure what kind of consequences this would have for contrib modules implementing hook_image_default_styles and hook_image_styles_alter which allow modules to assign effects to an image style. What happens if the effect doesn't actually exist?

Someone should probably test this out.

Please fix the whitespace issues in the patch. if ($foo) instead of if($foo).

Changed to Drupal coding standards.

Sorry, it was my syntax mistake. Check out new patch.

I think this needs to be fixed in 8.x first and then backport to D7.

In Drupa 8 it has been fixed already:

  public function rotate(ImageInterface $image, $degrees, $background = NULL) {
    // PHP installations using non-bundled GD do not have imagerotate.
    if (!function_exists('imagerotate')) {
      watchdog('image', 'The image %file could not be rotated because the imagerotate() function is not available in this PHP installation.', array('%file' => $image->getSource()));
      return FALSE;
    }
    //--
  }

This has been fixed in recent drupal 7.x branch.