Add decoding and fetchpriority to img allowed attrs

These performance attributes are commonly added by WordPress
core (wp_img_tag_add_loading_optimization_attrs) and should
not be stripped when content passes through KSES filtering.

Author: adamsilverstein

src/wp-includes/kses.php

@@ -210,19 +210,21 @@
 		),
 		'i'          => array(),
 		'img'        => array(
-			'alt'      => true,
-			'align'    => true,
-			'border'   => true,
-			'height'   => true,
-			'hspace'   => true,
-			'loading'  => true,
-			'longdesc' => true,
-			'vspace'   => true,
-			'src'      => true,
-			'srcset'   => true,
-			'usemap'   => true,
-			'width'    => true,
-			'sizes'    => true,
+			'alt'           => true,
+			'align'         => true,
+			'border'        => true,
+			'decoding'      => true,
+			'fetchpriority' => true,
+			'height'        => true,
+			'hspace'        => true,
+			'loading'       => true,
+			'longdesc'      => true,
+			'vspace'        => true,
+			'src'           => true,
+			'srcset'        => true,
+			'usemap'        => true,
+			'width'         => true,
+			'sizes'         => true,
 		),
 		'ins'        => array(
 			'datetime' => true,