Move to new best practice of defining $content_width

See: Automattic/_s/pull/460

functions.php

@@ -5,13 +5,6 @@
  * @package Decode
  */
 
-/*
- * Set the content width based on the theme's design and stylesheet.
- */
-if ( ! isset( $content_width ) ) {
-	$content_width = 640; /* pixels */
-}
-
 if ( ! function_exists( 'decode_setup' ) ) :
 /*
  * Sets up theme defaults and registers support for various WordPress features.
@@ -122,6 +115,18 @@ function decode_setup() {
 endif; // decode_setup
 add_action( 'after_setup_theme', 'decode_setup' );
 
+/**
+ * Set the content width in pixels, based on the theme's design and stylesheet.
+ *
+ * Priority 0 to make it available to lower priority callbacks.
+ *
+ * @global int $content_width
+ */
+function decode_content_width() {
+	$GLOBALS['content_width'] = apply_filters( 'decode_content_width', 640 );
+}
+add_action( 'after_setup_theme', 'decode_content_width', 0 );
+
 /*
  * Register styles and scripts.
  */