Fixing zero elevation bug when concave hull overlap was not detected.

Backported from tm_perf_optims

src/libslic3r/MTUtils.hpp

@@ -320,6 +320,9 @@ using FloatingOnly = enable_if_t<std::is_floating_point<T>::value, O>;
 template<class T, class O = T>
 using ScaledCoordOnly = enable_if_t<is_scaled_coord<T>::value, O>;
 
+template<class T, class O = T>
+using IntegerOnly = enable_if_t<std::is_integral<T>::value, O>;
+
 template<class T, class O = T>
 using ArithmeticOnly = enable_if_t<std::is_arithmetic<T>::value, O>;
 
@@ -384,6 +387,13 @@ unscaled(const Eigen::Matrix<Tin, N, EigenArgs...> &v) noexcept
     return v.template cast<Tout>() * SCALING_FACTOR;
 }
 
+template<class T> inline std::vector<T> reserve_vector(size_t capacity)
+{
+    std::vector<T> ret;
+    ret.reserve(capacity);
+    return ret;
+}
+
 } // namespace Slic3r
 
 #endif // MTUTILS_HPP