Added out of bounds check for image patch offset.  Also renamed to xOffset, yOffset to be consistent with usage in this imageio file.
Added out of bounds check for image patch offset.  Also renamed to xOffset, yOffset to be consistent with usage in this imageio file.

git-svn-id: 602a26e73ae9f2a14e2bb95676f2e137fcc39919@2819 7c0444ca-5550-0410-9008-faecdbe78eb5

PetaVision/OpenPV,dpaiton/OpenPV,PetaVision/OpenPV,dpaiton/OpenPV,PetaVision/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV,PetaVision/OpenPV,PetaVision/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV