Introduce CPDFPageFromFPDFPage()

fpdfsdk/src/fpdf_flatten.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 "../../public/fpdf_flatten.h"
 #include "../include/fsdk_define.h"
 
 typedef CFX_ArrayTemplate<CPDF_Dictionary*> CPDF_ObjectArray;
@@ skipping 292 matching lines @@
   FX_FLOAT left = FX_MIN(FX_MIN(x1, x2), FX_MIN(x3, x4));
   FX_FLOAT bottom = FX_MIN(FX_MIN(y1, y2), FX_MIN(y3, y4));
 
   fa = (rcAnnot.right - rcAnnot.left) / fStreamWidth;
   fd = (rcAnnot.top - rcAnnot.bottom) / fStreamHeight;
   fe = rcAnnot.left - left * fa;
   ff = rcAnnot.bottom - bottom * fd;
 }
 
 DLLEXPORT int STDCALL FPDFPage_Flatten(FPDF_PAGE page, int nFlag) {
+  CPDF_Page* pPage = CPDFPageFromFPDFPage(page);
   if (!page) {
     return FLATTEN_FAIL;
   }
 
-  CPDF_Page* pPage = (CPDF_Page*)(page);
   CPDF_Document* pDocument = pPage->m_pDocument;
   CPDF_Dictionary* pPageDict = pPage->m_pFormDict;
 
   if (!pDocument || !pPageDict) {
     return FLATTEN_FAIL;
   }
 
   CPDF_ObjectArray ObjectArray;
   CPDF_RectArray RectArray;
 
@@ skipping 189 matching lines @@
     pNewXObject->SetData((const uint8_t*)sStream, sStream.GetLength(), FALSE,
                          FALSE);
   }
   pPageDict->RemoveAt("Annots");
 
   ObjectArray.RemoveAll();
   RectArray.RemoveAll();
 
   return FLATTEN_SUCCESS;
 }