Fix for a 32 vs 64 bit bug preventing image transforms from working properly.

Hi Mojca, 

this is a fix for a bug I found when looking into a comment made by Ethan over at the gnuplot site.

BTW, I looked over the sources with an eye to clean up the code and modernize it some time ago. In the process I started https://github.com/persquare/aquaterm2 which might some day become a replacement for the aging original AquaTerm. Most stuff is working and with much less (and likely easier to understand) code. It is however 10.7+ only. Try it out if you like, the API should be the same.