Embeds: Introduce `embed_thumbnail_id` filter for thumbnail image ID in the embed template.

Props ketuchetan, Dency, jontyravi, Ov3rfly.
Fixes #39759.
Built from https://develop.svn.wordpress.org/trunk@41565


git-svn-id: http://core.svn.wordpress.org/trunk@41398 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme-compat/embed-content.php

@@ -22,6 +22,15 @@
 			$thumbnail_id = get_the_ID();
 		}
 
+		/**
+		 * Filters the thumbnail image ID for use in the embed template.
+		 *
+		 * @since 4.9.0
+		 *
+		 * @param int $thumbnail_id Attachment ID.
+		 */
+		$thumbnail_id = apply_filters( 'embed_thumbnail_id', $thumbnail_id );
+
 		if ( $thumbnail_id ) {
 			$aspect_ratio = 1;
 			$measurements = array( 1, 1 );

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41564';
+$wp_version = '4.9-alpha-41565';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.