Changed return type of Image::operator() const to const T&.

This is a very small pull request, but since static_image.h already does this, I thought there might be a reason why Halide::Image::operator () doesn't return a const reference.

This change enables Halide client code to take the address-of elements of const images, like they already can with non-const images. Of course, the pointer is still const, but this enables easy/efficient memcpy'ing from rows/slices of Images.