Media: Don't use 'full' as array key in `wp_calculate_image_srcset()`.

In `wp_calculate_image_srcset()` we get an array of image sizes
associated with an attachment and then add the original image's
information to the array before processing the `srcset`. In doing
so, we set the original data to a `$image_sizes['full']` key, which
could stomp on any custom image sizes using `full` as a size name.

This avoid the issues by adding the original data without a named
key, which is never referenced anyway.

Props jaspermdegroot.
Fixes #36345.

git-svn-id: https://develop.svn.wordpress.org/trunk@37986 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -1008,7 +1008,7 @@ function wp_calculate_image_srcset( $size_array, $image_src, $image_meta, $attac
 	 * If src is an intermediate size GIF, the full size is excluded from srcset to keep a flattened GIF from becoming animated.
 	 */
 	if ( ! isset( $image_sizes['thumbnail']['mime-type'] ) || 'image/gif' !== $image_sizes['thumbnail']['mime-type'] ) {
-		$image_sizes['full'] = array(
+		$image_sizes[] = array(
 			'width'  => $image_meta['width'],
 			'height' => $image_meta['height'],
 			'file'   => $image_basename,