Fix inconsistency in spherical Intrinsic_Spherical projection functor

[rjanvier]

There is an inconsistency between the way spherical projection is computed in the ceres cost functor and the way it's defined in Camera_Spherical.hpp. In the ceres functor the coordinates are shifted by -0.5 (top left is (0,0)) and in the Camera_Spherical.hpp file they are not (top left is (-0.5,-0.5)). Apparently there are some other inconsistencies of this kind in the lib that might need to be addressed.