Themes: Add a hook to filter theme header image URL.

This changeset introduces the `get_header_image` filter, which can be used to modify header image URL returned by `get_header_image()`, in themes that support the Header Image feature. 

Props hztyfoon, audrasjb, mukesh27, SergeyBiryukov, costdev.
Fixes #56180.

Built from https://develop.svn.wordpress.org/trunk@53741


git-svn-id: http://core.svn.wordpress.org/trunk@53300 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -1179,6 +1179,20 @@ function get_header_image() {
 		$url = get_random_header_image();
 	}
 
+	/**
+	 * Filters the header image URL.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @param string $url Header image URL.
+	 */
+	$url = apply_filters( 'get_header_image', $url );
+
+	if ( ! is_string( $url ) ) {
+		return false;
+	}
+
+	$url = trim( $url );
 	return sanitize_url( set_url_scheme( $url ) );
 }
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53740';
+$wp_version = '6.1-alpha-53741';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.