Introduce CPDFPageFromFPDFPage()

fpdfsdk/src/fpdf_flatten.cpp

Index: fpdfsdk/src/fpdf_flatten.cpp
diff --git a/fpdfsdk/src/fpdf_flatten.cpp b/fpdfsdk/src/fpdf_flatten.cpp
index 210df63d3e7e15b5baf14d1817d60df8a0fcced2..b2303f36aa0758f87b9a0e7177a1a5495fbf1800 100644
--- a/fpdfsdk/src/fpdf_flatten.cpp
+++ b/fpdfsdk/src/fpdf_flatten.cpp
@@ -310,11 +310,11 @@ void GetOffset(FX_FLOAT& fa,
 }
 
 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;