Make CPDF_Pattern::pattern_to_form() return a const-reference.

Make it a const method as well.

Change-Id: Ie149de369d730350a3e3fa4be2a21d1de410f8ab
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/55772
Auto-Submit: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fpdfapi/page/cpdf_pattern.h b/core/fpdfapi/page/cpdf_pattern.h
index 474575c..3882b81 100644
--- a/core/fpdfapi/page/cpdf_pattern.h
+++ b/core/fpdfapi/page/cpdf_pattern.h
@@ -31,7 +31,7 @@
   // All the getters that return pointers return non-NULL pointers.
   CPDF_Document* document() const { return m_pDocument.Get(); }
   CPDF_Object* pattern_obj() const { return m_pPatternObj.Get(); }
-  CFX_Matrix* pattern_to_form() { return &m_Pattern2Form; }
+  const CFX_Matrix& pattern_to_form() const { return m_Pattern2Form; }
   const CFX_Matrix& parent_matrix() const { return m_ParentMatrix; }
 
  protected:
diff --git a/core/fpdfapi/render/cpdf_renderstatus.cpp b/core/fpdfapi/render/cpdf_renderstatus.cpp
index 9d89dc4..29c9546 100644
--- a/core/fpdfapi/render/cpdf_renderstatus.cpp
+++ b/core/fpdfapi/render/cpdf_renderstatus.cpp
@@ -945,7 +945,7 @@
   bitmap_device.Attach(pBitmap, false, nullptr, false);
   pBitmap->Clear(0);
   CFX_FloatRect cell_bbox =
-      pPattern->pattern_to_form()->TransformRect(pPattern->bbox());
+      pPattern->pattern_to_form().TransformRect(pPattern->bbox());
   cell_bbox = mtObject2Device.TransformRect(cell_bbox);
   CFX_FloatRect bitmap_rect(0.0f, 0.0f, (float)width, (float)height);
   CFX_Matrix mtAdjust;
@@ -2126,7 +2126,7 @@
   if (rect.IsEmpty())
     return;
 
-  CFX_Matrix matrix = *pattern->pattern_to_form() * mtObj2Device;
+  CFX_Matrix matrix = pattern->pattern_to_form() * mtObj2Device;
   int alpha =
       FXSYS_round(255 * (bStroke ? pPageObj->m_GeneralState.GetStrokeAlpha()
                                  : pPageObj->m_GeneralState.GetFillAlpha()));
@@ -2164,7 +2164,7 @@
   if (clip_box.IsEmpty())
     return;
 
-  CFX_Matrix mtPattern2Device = *pPattern->pattern_to_form() * mtObj2Device;
+  CFX_Matrix mtPattern2Device = pPattern->pattern_to_form() * mtObj2Device;
 
   bool bAligned =
       pPattern->bbox().left == 0 && pPattern->bbox().bottom == 0 &&