lazy stroke preview rendering

src/util.h

@@ -198,6 +198,14 @@ public:
         }
         get_cv.notify_one();
     }
+    void Remove(T pkt)
+    {
+        std::unique_lock<std::mutex> lock(mutex);
+        auto it = std::find_if(q.begin(), q.end(), [&pkt](auto const& x) { return x.first == pkt; });
+        if (it != q.end())
+            q.erase(it);
+        if (Max > 0) post_cv.notify_all();
+    }
     T Get()
     {
         static T emptyT{};