Undefined array key "multiplier" + DivisionByZero when retina derivative is generated outside the download controller

Problem/Motivation

When a retina derivative is generated outside RetinaImageStyleDownloadController (e.g., programmatic ImageStyle::createDerivative()) two errors are thrown:

Warning: Undefined array key "multiplier" in auto_retina_auto_retina_effect_alter() (auto_retina.module:104)

DivisionByZeroError in Drupal\auto_retina\Service\AutoRetina->optimizeImageSize() (AutoRetina.php:249)

The point is that auto_retina_auto_retina_create_derivative_alter() reads $style->getThirdPartySettings('auto_retina') and forwards it to the effect alter. Those settings are populated only by AutoRetina::prepareStyle(), which runs only on the retina download route. On every other code path the array is empty, so multiplier is missing and is passed as null into optimizeImageSize(), which then divides by zero.

Steps to reproduce

1. Configure auto_retina with the default @2x suffix.
2. Create an image style with a width/height effect.
3. Call $style->createDerivative() directly with a @2x destination.

Proposed resolution

When the third-party settings are missing multiplier, call AutoRetina::prepareStyle($style, $destination) to derive them from the destination path before invoking the effect alter. Patch attached.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.