- 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

i.php

@@ -519,7 +519,7 @@ if (!$changes)
   ierror( $page['src_url'], 301);
 }
 
-if ($d_size[0]*$d_size[1] < 100000)
+if ($d_size[0]*$d_size[1] < 256000)
 {// strip metadata for small images
   $image->strip();
 }