Fallback image style for responsive image styles have been moved from instance settings to the responsive image style entity

The fallback image styles for responsive image styles have been moved from field instance settings to the responsive image style entity.

Any existing sites using responsive images in Drupal 8 should edit their responsive image mappings to include the fallback style, as the field formatter and theme functions no longer allow you to define a fallback image style outside of the responsive image mapping.

This simplifies responsive image configuration, as the fallback style only needs to be defined once on the mapping, rather than on every field where that mapping is used.

Before:

After:

Instance settings form:

Responsive Image Style form:

Developer can now set and get the fallback image style of a responsive image style:

$responsive_image_style = ResponsiveImageStyle::load('my_responsive_style');
// Get the fallback image style.
$fallback_image_style = $responsive_image_style->getFallbackImageStyle();

// Set the fallback image style.
$responsive_image_style->setFallbackImageStyle('thumbnail');