Fix rare GD lib bug

Fixed an issue where GD was present but did not always have the
imageconvolution function available.

Changed $mobile_first default to FALSE to avoid issues with
unpredictable browser behaviour relating to race conditions between
cookie setting and image requesting

adaptive-images.php

@@ -1,6 +1,6 @@
 <?php
 /* PROJECT INFO --------------------------------------------------------------------------------------------------------
-                Version: 1.3.3
+                Version: 1.3.4
                 Changelog: http://adaptive-images.com/changelog.txt
 
                 Homepage: http://adaptive-images.com