better alpha test

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@20195 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/image/image.cpp

@@ -286,7 +286,7 @@ public:
                 p.Red() = r;
                 p.Green() = g;
                 p.Blue() = b;
-                p.Alpha() = x;
+                p.Alpha() = (x*255.)/REAL_SIZE;
 
                 ++p; // same as p.OffsetX(1)
             }