Fixes https://github.com/Itseez/opencv_contrib/issues/332

Relevant material:
http://stackoverflow.com/questions/6066273/pointer-into-vector-but-not-into-a-valarray
http://objectmix.com/c/40001-address-first-member-valarray.html
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#389

I have to admit my naivety here, this issue is was complex for me
to diagnose. It appears that in the MSVC 2008/2010 implementation
of std::valarray, when indexing and assigning to a const variable,
that the const version of the member function
valarray<T>::operator[](size_t) returns a value instead of a
const-reference. Therefore, taking the address of this is a
compiler error as you can't take the address of an lvalue. By
performing a const_cast on the array we change which function
is called and thus we actually do recieve a const-reference.
This should not effect other compilers.