Use premultiplied alpha pixel format to match what is used everywhere else in wxMSW.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@49370 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/graphics.cpp

@@ -1140,7 +1140,7 @@ void wxGDIPlusContext::DrawBitmap( const wxBitmap &bmp, wxDouble x, wxDouble y,
                 helper->GetPixelFormat(),&data);
 
             image = new Bitmap(data.Width, data.Height, data.Stride,
-                PixelFormat32bppARGB , (BYTE*) data.Scan0);
+                PixelFormat32bppPARGB , (BYTE*) data.Scan0);
 
             helper->UnlockBits(&data);
         }