Add a pre_set_theme_mod_$name filter to set_theme_mod().

This is modeled after pre_update_option_$option in update_option() and pre_set_transient_$transient in set_transient().

props obenland.
fixes #14721.

Built from https://develop.svn.wordpress.org/trunk@27393


git-svn-id: http://core.svn.wordpress.org/trunk@27241 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -913,8 +913,21 @@ function get_theme_mod( $name, $default = false ) {
  */
 function set_theme_mod( $name, $value ) {
 	$mods = get_theme_mods();
+	$old_value = $mods[ $name ];
 
-	$mods[ $name ] = $value;
+	/**
+	 * Filter the theme mod value on save.
+	 *
+	 * The dynamic portion of the hook name, $name, refers to the key name of
+	 * the modification array. For example, 'header_textcolor', 'header_image',
+	 * and so on depending on the theme options.
+	 *
+	 * @since 3.9.0
+	 *
+	 * @param string $value     The new value of the theme mod.
+	 * @param string $old_value The current value of the theme mod.
+	 */
+	$mods[ $name ] = apply_filters( "pre_set_theme_mod_$name", $value, $old_value );
 
 	$theme = get_option( 'stylesheet' );
 	update_option( "theme_mods_$theme", $mods );