Add reserve_vector to libslic3r.h to be globally usable.

src/libslic3r/libslic3r.h

@@ -266,6 +266,15 @@ using IteratorOnly = std::enable_if_t<
     !std::is_same_v<typename std::iterator_traits<T>::value_type, void>, O
 >;
 
+template<class T, class I, class... Args> // Arbitrary allocator can be used
+IntegerOnly<I, std::vector<T, Args...>> reserve_vector(I capacity)
+{
+    std::vector<T, Args...> ret;
+    if (capacity > I(0)) ret.reserve(size_t(capacity));
+
+    return ret;
+}
+
 } // namespace Slic3r
 
 #endif