Fix -Wnon-virtual-dtor compiler warnings.

core/include/fpdfapi/fpdf_render.h

 // 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
 
 #ifndef _FPDF_RENDER_
 #define _FPDF_RENDER_
 #ifndef _FPDF_PAGE_
 #include "fpdf_page.h"
@@ skipping 120 matching lines @@
     FX_BOOL                                     m_bFirstLayer;
 
     void                        Render(CFX_RenderDevice* pDevice, const CPDF_PageObject* pStopObj,
                            const CPDF_RenderOptions* pOptions, const CFX_AffineMatrix* pFinalMatrix);
     friend class CPDF_RenderStatus;
     friend class CPDF_ProgressiveRenderer;
 };
 class IPDF_BackgroundDraw
 {
 public:
-
+    virtual ~IPDF_BackgroundDraw() { }
     virtual     void    OnDrawBackground(
         CFX_RenderDevice* pBitmapDevice,
         const CFX_AffineMatrix* pOriginal2Bitmap
     ) = 0;
 };
 class CPDF_ProgressiveRenderer : public CFX_Object
 {
 public:
 
     CPDF_ProgressiveRenderer();
@@ skipping 157 matching lines @@
 };
 class CPDF_RenderConfig : public CFX_Object
 {
 public:
     CPDF_RenderConfig();
     ~CPDF_RenderConfig();
     int                                 m_HalftoneLimit;
     int                                 m_RenderStepLimit;
 };
 #endif