Add a small LRU cache for the JBIG2 symbol dictionary.

core/src/fxcodec/jbig2/JBig2_SymbolDict.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
  
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "JBig2_SymbolDict.h"
 CJBig2_SymbolDict::CJBig2_SymbolDict()
 {
     SDNUMEXSYMS = 0;
     SDEXSYMS = NULL;
     m_bContextRetained = FALSE;
     m_gbContext = m_grContext = NULL;
 }
 
+CJBig2_SymbolDict *CJBig2_SymbolDict::DeepCopy()
+{
+    CJBig2_SymbolDict *dst = NULL;
+    CJBig2_SymbolDict *src = this;
+    if (src->m_bContextRetained ||
+        src->m_gbContext ||
+        src->m_grContext) {
+        return NULL;
+    }
+    JBIG2_ALLOC(dst, CJBig2_SymbolDict());
+    dst->SDNUMEXSYMS = src->SDNUMEXSYMS;
+    dst->SDEXSYMS = (CJBig2_Image**)m_pModule->JBig2_Malloc2(
+        sizeof(CJBig2_Image*), src->SDNUMEXSYMS);
+    for(FX_DWORD i = 0; i < src->SDNUMEXSYMS; i++) {
+        JBIG2_ALLOC(dst->SDEXSYMS[i],
+                    CJBig2_Image(*(src->SDEXSYMS[i])));
+    }
+    return dst;
+}
+
 CJBig2_SymbolDict::~CJBig2_SymbolDict()
 {
     if(SDEXSYMS) {
         for(FX_DWORD i = 0; i < SDNUMEXSYMS; i++) {
             if(SDEXSYMS[i]) {
                 delete SDEXSYMS[i];
             }
         }
         m_pModule->JBig2_Free(SDEXSYMS);
     }
     if(m_bContextRetained) {
         if(m_gbContext) {
             m_pModule->JBig2_Free(m_gbContext);
         }
         if(m_grContext) {
             m_pModule->JBig2_Free(m_grContext);
         }
     }
 }