- derivative fix floating point rounding errors for certain sizes

- increase derivative size for which metadata is kept
- imagick use 4:2:2 chroma subsampling (reduce file size by 20-30% with "almost" no human perception)
- dont show mobile theme links if there is no mobile theme installed
- mobile theme favicon removed because 404

git-svn-id: http://piwigo.org/svn/trunk@13426 68402e56-0260-453c-a942-63ccdbb3a9ee

include/page_tail.php

@@ -66,7 +66,7 @@ if ($conf['show_gt'])
 $template->assign('debug', $debug_vars );
 
 //------------------------------------------------------------- mobile version
-if (get_device() != 'desktop')
+if ( !empty($conf['mobile_theme']) && get_device() != 'desktop')
 {
   $template->assign('TOGGLE_MOBILE_THEME_URL',
       add_url_params(