Media: Enable lazy-loading of images by automatically adding the new loading="lazy" attribute to image tags on the front-end.

- Introduces `wp_lazy_loading_enabled()`, `wp_filter_content_tags()`, `wp_img_tag_add_loading_attr()`, and `wp_img_tag_add_srcset_and_sizes_attr()` functions.
- Introduces `wp_lazy_loading_enabled`, `wp_img_tag_add_loading_attr`, and `wp_img_tag_add_srcset_and_sizes_attr` filters.

Props flixos90, addyosmani, mor10, swissspidy, pierlo, westonruter, spacedmonkey, mikeschroder, jonoaldersonwp, peterwilsoncc, narwen, jeffpaul, OptimizingMatters, futtta, mukeshpanchal27, azaozz.

Fixes #44427.

git-svn-id: https://develop.svn.wordpress.org/trunk@47554 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/pluggable.php

@@ -2606,6 +2606,8 @@ if ( ! function_exists( 'get_avatar' ) ) :
 	 *                                       Default null.
 	 *     @type bool         $force_display Whether to always show the avatar - ignores the show_avatars option.
 	 *                                       Default false.
+	 *     @type string       $loading       Value for the `loading` attribute.
+	 *                                       Default null.
 	 *     @type string       $extra_attr    HTML attributes to insert in the IMG element. Is not sanitized. Default empty.
 	 * }
 	 * @return string|false `<img>` tag for the user's avatar. False on failure.
@@ -2623,9 +2625,14 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			'alt'           => '',
 			'class'         => null,
 			'force_display' => false,
+			'loading'       => null,
 			'extra_attr'    => '',
 		);
 
+		if ( wp_lazy_loading_enabled( 'img', 'get_avatar' ) ) {
+			$defaults['loading'] = 'lazy';
+		}
+
 		if ( empty( $args ) ) {
 			$args = array();
 		}
@@ -2695,6 +2702,18 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			}
 		}
 
+		// Add `loading` attribute.
+		$extra_attr = $args['extra_attr'];
+		$loading    = $args['loading'];
+
+		if ( in_array( $loading, array( 'lazy', 'eager' ), true ) && ! preg_match( '/\bloading\s*=/', $extra_attr ) ) {
+			if ( ! empty( $extra_attr ) ) {
+				$extra_attr .= ' ';
+			}
+
+			$extra_attr .= "loading='{$loading}'";
+		}
+
 		$avatar = sprintf(
 			"<img alt='%s' src='%s' srcset='%s' class='%s' height='%d' width='%d' %s/>",
 			esc_attr( $args['alt'] ),
@@ -2703,7 +2722,7 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			esc_attr( join( ' ', $class ) ),
 			(int) $args['height'],
 			(int) $args['width'],
-			$args['extra_attr']
+			$extra_attr
 		);
 
 		/**