Improve caching of lensfun mismatches

rtengine/rtlensfun.h

@@ -21,6 +21,7 @@
 #pragma once
 
 #include <memory>
+#include <set>
 #include <vector>
 
 #include <glibmm/ustring.h>
@@ -135,7 +136,7 @@ private:
     mutable MyMutex lfDBMutex;
     static LFDatabase instance_;
     lfDatabase *data_;
-    static Glib::ustring lastKey;
+    static std::set<std::string> notFound;
 };
 
 } // namespace rtengine