allow raw access to a part of the image only (faster when using alpha)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@20409 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/image/image.cpp

@@ -260,7 +260,9 @@ public:
     {
         SetClientSize(SIZE, SIZE);
 
-        wxAlphaPixelData data(m_bitmap);
+        wxAlphaPixelData data(m_bitmap,
+                              wxPoint(BORDER, BORDER),
+                              wxSize(REAL_SIZE, REAL_SIZE));
         if ( !data )
         {
             wxLogError(_T("Failed to gain raw access to bitmap data"));
@@ -271,8 +273,6 @@ public:
 
         wxAlphaPixelData::Iterator p(data);
 
-        p.Offset(data, BORDER, BORDER);
-
         for ( int y = 0; y < REAL_SIZE; ++y )
         {
             wxAlphaPixelData::Iterator rowStart = p;