Make sure we use ptrdiff_t for index into arrays.

This is a requirement since you can use negative indices into arrays.

vtkm/cont/internal/IteratorFromArrayPortal.h

@@ -107,7 +107,7 @@ public:
 
   VTKM_CONT_EXPORT
   detail::IteratorFromArrayPortalValue<ArrayPortalType>
-  operator[](std::size_t idx) const
+  operator[](std::ptrdiff_t idx) const //NEEDS to be signed
   {
     return detail::IteratorFromArrayPortalValue<ArrayPortalType>(this->Portal,
            this->Index + static_cast<vtkm::Id>(idx) );

vtkm/exec/cuda/internal/IteratorFromArrayPortal.h

@@ -113,7 +113,7 @@ public:
 
   VTKM_EXEC_EXPORT
   PortalValue<ArrayPortalType>
-  operator[](std::size_t idx) const
+  operator[](std::ptrdiff_t idx) const //NEEDS to be signed
   {
     return PortalValue<ArrayPortalType>(this->Portal,
            this->Index + static_cast<vtkm::Id>(idx) );