Fix definitions of `type_min` and `type_zero` in `xr_types.h`

Created by: AMS21

While looking at the code inside xr_types.h I've notices some very strange definitions which are used later in the code to define constants.

// Type limits
template <typename T>
constexpr auto type_max = std::numeric_limits<T>::max();

template <typename T>
constexpr auto type_min = -std::numeric_limits<T>::max();

template <typename T>
constexpr auto type_zero = std::numeric_limits<T>::min();

type_min is not the minimum value of integer types it is actually the minimum + 1. This only works for floating point types. type_zero is the actual minimum of a type. But is only 0 for unsigned types. For signed types its a negative value.

Thus we have some very strange constants.

constexpr int int_min = type_min<int>;         // actually -2147483647 not -2147483648
constexpr int int_zero = type_zero<int>;       // actually -2147483648 not 0
...
constexpr float flt_zero = type_zero<float>;   // actually 1.175494e-38 not 0.0
...
constexpr double dbl_zero = type_zero<double>; // actually 2.225074e-308 not 0.0

Fixes #195