Current implementation removes element when looping.
The efficiency is not so good.

The codes below, offered by inter engineer zhuoshi sun, will improve performance.

 //< modified by sunzhuoshi
     if (m_pTextures->begin())
      {
         CCTexture2D *texture = NULL;
         std::string key;
         std::list keysToRemove;
          while (texture = m_pTextures->next(&key))
          {
             if (texture->retainCount() == 1)
              {
                 keysToRemove.push_back(key);
             }
         };
         m_pTextures->end();
         for (std::list::iterator it=keysToRemove.begin(); it!=keysToRemove.end(); ++it)          {
             CCLOG("cocos2d: CCTextureCache: removing unused texture: %s", (*it).c_str());             
             m_pTextures->removeObjectForKey(*it);
         }
     }     //>

The codes should be integrated into gles20 branch.