Fix huge performance problem in wxStdInputStream when using MSVC8/9.

Our overridden xsgetn() method was never called when using these compilers
because they used their own, non-standard, _Xsgetn_s() instead. Because of
this the stream was always read character by character which was very
inefficient.

Fix the problem by overriding _Xsgetn_s() for these compilers and explicitly
forwarding it to xsgetn().

Closes #13926.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70515 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -464,6 +464,7 @@ All:
 - Added "%z" support to wxDateTime::Format() and Parse() (Armel Asselin).
 - Add wxHTTP::SetPostBuffer(wxMemoryBuffer) and SetPostText() (Eran Ifrah).
 - Fix wrong time zone used in wxDateTime::UNow().
+- Fix performance of wxStdInputStream with MSVC8/9 (wsu).
 
 All (GUI):
 

include/wx/stdstream.h

@@ -47,6 +47,18 @@ protected:
     virtual int uflow();
     virtual int pbackfail(int c = EOF);
 
+    // Special work around for VC8/9 (this bug was fixed in VC10 and later):
+    // these versions have non-standard _Xsgetn_s() that it being called from
+    // the stream code instead of xsgetn() and so our overridden implementation
+    // never actually gets used. To work around this, forward to it explicitly.
+#if defined(__VISUALC8__) || defined(__VISUALC9__)
+    virtual std::streamsize
+    _Xsgetn_s(char *s, size_t WXUNUSED(size), std::streamsize n)
+    {
+        return xsgetn(s, n);
+    }
+#endif // VC8 or VC9
+
     wxInputStream& m_stream;
     int m_lastChar;
 };