Avoid using auto with Vc operator[int] accessor

When accessing the scalar entry in a Vc SIMD floating point value, avoid using the auto type specifier

const auto xLoc = pixPtnLoc.X()[i];

as this can generate an 'unexpected' type reference to an internal Vc wrapper type.

Instead explicitly force cast to double.

exposed, for some reason, only in the v3 optimized builds (e.g. x86_64_v3-centos7-gcc11+dd4hep-opt).