Fix -Wnon-virtual-dtor compiler warnings.

core/include/fxge/fx_dib.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_DIB_H_
 #define _FPDF_DIB_H_
 #ifndef _FXCRT_EXTENSION_
 #include "../fxcrt/fx_ext.h"
@@ skipping 384 matching lines @@
 
     const CFX_DIBSource*        m_pSrc;
 
     FX_BOOL                                     m_bAutoDropSrc;
 
     FX_LPBYTE                           m_pScanline;
 };
 class IFX_ScanlineComposer
 {
 public:
+    virtual ~IFX_ScanlineComposer() { }
 
     virtual     void            ComposeScanline(int line, FX_LPCBYTE scanline, FX_LPCBYTE scan_extra_alpha = NULL) = 0;
 
-
     virtual FX_BOOL             SetInfo(int width, int height, FXDIB_Format src_format, FX_DWORD* pSrcPalette) = 0;
 };
 class CFX_ScanlineCompositor : public CFX_Object
 {
 public:
-
     CFX_ScanlineCompositor();
 
     ~CFX_ScanlineCompositor();
 
     FX_BOOL                             Init(FXDIB_Format dest_format, FXDIB_Format src_format, FX_INT32 width, FX_DWORD* pSrcPalette,
                              FX_DWORD mask_color, int blend_type, FX_BOOL bClip, FX_BOOL bRgbByteOrder = FALSE, int alpha_flag = 0, void* pIccTransform = NULL);
 
 
     void                                CompositeRgbBitmapLine(FX_LPBYTE dest_scan, FX_LPCBYTE src_scan, int width, FX_LPCBYTE clip_scan,
             FX_LPCBYTE src_extra_alpha = NULL, FX_LPBYTE dst_extra_alpha = NULL);
@@ skipping 172 matching lines @@
     int                                 m_Status;
     int                                 m_DestLeft, m_DestTop;
     FX_RECT                             m_ClipBox;
     FX_DWORD                    m_Flags;
     int                                 m_AlphaFlag;
     void*                               m_pIccTransform;
     FX_BOOL                             m_bRgbByteOrder;
     int                                 m_BlendType;
 };
 #endif