draw main brush preview first if the queue is long

src/util.h

@@ -173,7 +173,7 @@ public:
         q.push_back(pkt);
         get_cv.notify_one();
     }
-    void PostUnique(T pkt)
+    void PostUnique(T pkt, bool top)
     {
         std::unique_lock<std::mutex> lock(mutex);
         if (Max > 0)
@@ -182,7 +182,7 @@ public:
             if (q.size() >= Max) return;
         }
         if (std::find(q.begin(), q.end(), pkt) == q.end());
-            q.push_back(pkt);
+            top ? q.push_front(pkt) : q.push_back(pkt);
         get_cv.notify_one();
     }
     T Get()