Actually delegate to Bitmap's move assign op

Change-Id: Ib57906ead469d64d324f3820c6f5d3c49ab4cd0a

include/vcl/alpha.hxx

@@ -20,6 +20,10 @@
 #ifndef INCLUDED_VCL_ALPHA_HXX
 #define INCLUDED_VCL_ALPHA_HXX
 
+#include <sal/config.h>
+
+#include <utility>
+
 #include <vcl/dllapi.h>
 #include <vcl/bitmap.hxx>
 
@@ -40,7 +44,7 @@ public:
 
     AlphaMask&  operator=( const Bitmap& rBitmap );
     AlphaMask&  operator=( const AlphaMask& rAlphaMask ) { return static_cast<AlphaMask&>( Bitmap::operator=( rAlphaMask ) ); }
-    AlphaMask&  operator=( AlphaMask&& rAlphaMask ) { return static_cast<AlphaMask&>( Bitmap::operator=( rAlphaMask ) ); }
+    AlphaMask&  operator=( AlphaMask&& rAlphaMask ) { return static_cast<AlphaMask&>( Bitmap::operator=( std::move(rAlphaMask) ) ); }
     bool        operator!() const { return Bitmap::operator!(); }
     bool        operator==( const AlphaMask& rAlphaMask ) const { return Bitmap::operator==( rAlphaMask ); }
     bool        operator!=( const AlphaMask& rAlphaMask ) const { return Bitmap::operator!=( rAlphaMask ); }