Incorporating performance optimizations from libnest2d

xs/src/libnest2d/libnest2d/clipper_backend/clipper_backend.cpp

@@ -1,15 +1,35 @@
 #include "clipper_backend.hpp"
+#include <atomic>
 
 namespace libnest2d {
 
 namespace  {
+
+class SpinLock {
+    std::atomic_flag& lck_;
+public:
+
+    inline SpinLock(std::atomic_flag& flg): lck_(flg) {}
+
+    inline void lock() {
+        while(lck_.test_and_set(std::memory_order_acquire)) {}
+    }
+
+    inline void unlock() { lck_.clear(std::memory_order_release); }
+};
+
 class HoleCache {
     friend struct libnest2d::ShapeLike;
 
     std::unordered_map< const PolygonImpl*, ClipperLib::Paths> map;
 
     ClipperLib::Paths& _getHoles(const PolygonImpl* p) {
+        static std::atomic_flag flg = ATOMIC_FLAG_INIT;
+        SpinLock lock(flg);
+
+        lock.lock();
         ClipperLib::Paths& paths = map[p];
+        lock.unlock();
 
         if(paths.size() != p->Childs.size()) {
             paths.reserve(p->Childs.size());