Responsive image format for media

Reworked the patch to use the file from the media source field instead of the thumbnail.

The ticket still needs tests for a new formatter.

Patch from #37 does not seem to apply to 9.5.3.

git apply -v --directory=web responsive-style-for-media-9.5-37.patch
Checking patch web/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php...
error: while searching for:
      ],
    ];

    $link_types = [
      'content' => $this->t('Content'),
      'file' => $this->t('File'),
    ];
    $elements['image_link'] = [
      '#title' => $this->t('Link image to'),
      '#type' => 'select',
      '#default_value' => $this->getSetting('image_link'),
      '#empty_option' => $this->t('Nothing'),
      '#options' => $link_types,
    ];

    return $elements;

error: patch failed: web/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php:148
error: web/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php: patch does not apply
Checking patch web/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveMediaImageFormatter.php...

Nice work, still needs tests as of #21, then it's ready for review.

Loading attribute is not set with that formatter!
It is missing 2 lines before $elements[$delta] is set:
When I find time I'll provide patch.

$image_loading_settings = $this->getSetting('image_loading');
$item_attributes['loading'] = $image_loading_settings['attribute'];

$elements[$delta] = [
.....