Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/filter/ElemHideEmulationFilter.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 10 matching lines @@
 
 #include "Filter.h"
 #include "ElemHideBase.h"
 
 class ElemHideEmulationFilter : public ElemHideBase
 {
 public:
   explicit ElemHideEmulationFilter(const String& text,
       const ElemHideData& data);
 };
+
+template<>
+inline ElemHideEmulationFilter* Filter::As<ElemHideEmulationFilter>()
+{
+  if ((mType & Type::ELEMHIDEEMULATION) == Type::ELEMHIDEEMULATION)
+    return nullptr;
+
+  return static_cast<ElemHideEmulationFilter*>(this);
+}