let backgrounds be put under SSL

README

@@ -1521,6 +1521,8 @@ dir: directory to write backgrounds too. Default is '/background/'
      subdir of install dir.
 path: path to backgrounds. Default is sub-path of install path; note
       that you may need to change this if you change site-path too.
+ssl: Whether or not to use HTTPS for background files. Defaults to
+     null, meaning to guess from site-wide SSL settings.
 
 ping
 ----

classes/Design.php

@@ -155,9 +155,20 @@ class Design extends Memcached_DataObject
             $server = common_config('site', 'server');
         }
 
-        // XXX: protocol
+        $ssl = common_config('background', 'ssl');
 
-        return 'http://'.$server.$path.$filename;
+        if (is_null($ssl)) { // null -> guess
+            if (common_config('site', 'ssl') == 'always' &&
+                !common_config('background', 'server')) {
+                $ssl = true;
+            } else {
+                $ssl = false;
+            }
+        }
+
+        $protocol = ($ssl) ? 'https' : 'http';
+
+        return $protocol.'://'.$server.$path.$filename;
     }
 
     function setDisposition($on, $off, $tile)

lib/default.php

@@ -116,7 +116,8 @@ $default =
         'background' =>
         array('server' => null,
               'dir' => INSTALLDIR . '/background/',
-              'path' => $_path . '/background/'),
+              'path' => $_path . '/background/',
+              'ssl' => null),
         'public' =>
         array('localonly' => true,
               'blacklist' => array(),