Media: Add filter for post thumbnail id.

Introduces new filter `post_thumbnail_id` which allows overriding the default id returned from `get_post_thumbnail_id()`.

Props engelen, alexvorn2, gilbitron, sebastianpisula, SergeyBiryukov, leogermani, rzen, joemcgill, audrasjb.
Fixes #23983.



git-svn-id: https://develop.svn.wordpress.org/trunk@52028 602fd350-edb4-49c9-b593-d223f7449a82

Author: antpb

src/wp-includes/post-thumbnail-template.php

@@ -57,7 +57,17 @@ function get_post_thumbnail_id( $post = null ) {
 		return false;
 	}
 
-	return (int) get_post_meta( $post->ID, '_thumbnail_id', true );
+	$thumbnail_id = (int) get_post_meta( $post->ID, '_thumbnail_id', true );
+
+	/**
+	 * Filters post thumbnail ID.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param int|false        $thumbnail_id Post thumbnail ID or false if the post does not exist.
+	 * @param int|WP_Post|null $post         Post ID or WP_Post object. Default is global `$post`.
+	 */
+	return (int) apply_filters( 'post_thumbnail_id', $thumbnail_id, $post );
 }
 
 /**