make AlphaMask constructors explicit

to reduce surprises in some changes I'm planning

Change-Id: I61cb58d51ab77f57e55159017c7bece5bd99953f
Reviewed-on: https://gerrit.libreoffice.org/54161Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

include/vcl/alpha.hxx

@@ -34,10 +34,10 @@ class VCL_DLLPUBLIC AlphaMask : private Bitmap
 public:
 
                 AlphaMask();
-                AlphaMask( const Bitmap& rBitmap );
+    explicit    AlphaMask( const Bitmap& rBitmap );
                 AlphaMask( const AlphaMask& rAlphaMask );
                 AlphaMask( AlphaMask&& rAlphaMask );
-                AlphaMask( const Size& rSizePixel, const sal_uInt8* pEraseTransparency = nullptr );
+    explicit    AlphaMask( const Size& rSizePixel, const sal_uInt8* pEraseTransparency = nullptr );
     virtual     ~AlphaMask() override;
 
     AlphaMask&  operator=( const Bitmap& rBitmap );

vcl/source/gdi/bitmapex.cxx

@@ -265,7 +265,7 @@ AlphaMask BitmapEx::GetAlpha() const
     }
     else
     {
-        return maMask;
+        return AlphaMask(maMask);
     }
 }