AlphaMask derives from Bitmap so their destructors should be virtual

(cherry picked from commit 846a7425)

Conflicts:
	vcl/inc/vcl/alpha.hxx
	vcl/inc/vcl/bitmap.hxx

Change-Id: I8bf83e2edde33f9aee50a7feffe18bcb5c352dd4

vcl/inc/vcl/alpha.hxx

@@ -47,7 +47,7 @@ public:
     AlphaMask( const Bitmap& rBitmap );
     AlphaMask( const AlphaMask& rAlphaMask );
     AlphaMask( const Size& rSizePixel, sal_uInt8* pEraseTransparency = NULL );
-    ~AlphaMask();
+    virtual ~AlphaMask();
 
     AlphaMask&  operator=( const Bitmap& rBitmap );
     AlphaMask&  operator=( const AlphaMask& rAlphaMask )

vcl/inc/vcl/bitmap.hxx

@@ -379,7 +379,7 @@ public:
     Bitmap( const Size& rSizePixel, sal_uInt16 nBitCount, const BitmapPalette* pPal = NULL );
     Bitmap( const ResId& rResId );
     Bitmap( SalBitmap* pSalBitmap );
-    ~Bitmap();
+    virtual ~Bitmap();
 
     Bitmap&                 operator=( const Bitmap& rBitmap );
     inline sal_Bool             operator!() const;