Update CGXP's blank.gif using the one found in OpenLayers

Google Maps base layers remain blank in any version of IE. @lbesson (from Camptocamp.org) has figured out that the ``blank.gif``file used in CGXP is somehow "corrupted" or incorrect. I have tried to replaced it by the file found in the very last version of OpenLayers and it solves the problem.

Some more discussions about this issue:
http://drupal.org/node/1364304