handle antialiased rendering as premultiplied

core/fpdfapi/render/cpdf_renderstatus.cpp

Index: core/fpdfapi/render/cpdf_renderstatus.cpp
diff --git a/core/fpdfapi/render/cpdf_renderstatus.cpp b/core/fpdfapi/render/cpdf_renderstatus.cpp
index 7c389ef37c664a08bdfda7b3f468dbf37b280237..8c708093d519d06e2ab0205951ddb79f6e6a9c7b 100644
--- a/core/fpdfapi/render/cpdf_renderstatus.cpp
+++ b/core/fpdfapi/render/cpdf_renderstatus.cpp
@@ -876,6 +876,9 @@ std::unique_ptr<CFX_DIBitmap> DrawPatternBitmap(
   CFX_FxgeDevice bitmap_device;
   bitmap_device.Attach(pBitmap.get(), false, nullptr, false);
   pBitmap->Clear(0);
+#if defined _SKIA_SUPPORT_PATHS_
+  pBitmap->SetIsOffscreen(true);
+#endif
   CFX_FloatRect cell_bbox = pPattern->bbox();
   pPattern->pattern_to_form()->TransformRect(cell_bbox);
   pObject2Device->TransformRect(cell_bbox);
@@ -893,6 +896,9 @@ std::unique_ptr<CFX_DIBitmap> DrawPatternBitmap(
   CPDF_RenderContext context(pDoc, pCache);
   context.AppendLayer(pPattern->form(), &mtPattern2Bitmap);
   context.Render(&bitmap_device, &options, nullptr);
+#if defined _SKIA_SUPPORT_PATHS_
+  pBitmap->UnPreMultiply();
+#endif
   return pBitmap;
 }
 
@@ -1516,6 +1522,9 @@ bool CPDF_RenderStatus::ProcessTransparency(CPDF_PageObject* pPageObj,
     return true;
   CFX_DIBitmap* bitmap = bitmap_device.GetBitmap();
   bitmap->Clear(0);
+#ifdef _SKIA_SUPPORT_PATHS_
+  bitmap->SetIsOffscreen(true);
+#endif
   CFX_Matrix new_matrix = *pObj2Device;
   new_matrix.TranslateI(-rect.left, -rect.top);
   new_matrix.Scale(scaleX, scaleY);