Make wxTextStream classes work with surrogates under MSW

On the platforms using UTF-16 for wchar_t we can't read nor write Unicode data
one wchar_t at a time as a single half of a surrogate character can't be
converted to or from the encoding of the stream.

To fix this, we may need to store the last wchar_t already read from the
stream but not returned yet in wxTextInputStream::NextChar() and store,
without writing it, the wchar_t passed to wxTextOutputStream::PutChar() until
the second half of the surrogate is written.

See #17070.

include/wx/txtstrm.h

@@ -87,7 +87,16 @@ protected:
 
 #if wxUSE_UNICODE
     wxMBConv *m_conv;
-#endif
+
+    // The second half of a surrogate character when using UTF-16 for wchar_t:
+    // we can't return it immediately from GetChar() when we read a Unicode
+    // code point outside of the BMP, but we can't keep it in m_lastBytes
+    // neither because it can't separately decoded, so we have a separate 1
+    // wchar_t buffer just for this case.
+#if SIZEOF_WCHAR_T == 2
+    wchar_t m_lastWChar;
+#endif // SIZEOF_WCHAR_T == 2
+#endif // wxUSE_UNICODE
 
     bool   EatEOL(const wxChar &c);
     void   UngetLast(); // should be used instead of wxInputStream::Ungetch() because of Unicode issues
@@ -165,7 +174,13 @@ protected:
 
 #if wxUSE_UNICODE
     wxMBConv *m_conv;
-#endif
+
+#if SIZEOF_WCHAR_T == 2
+    // The first half of a surrogate character if one was passed to PutChar()
+    // and couldn't be output when it was called the last time.
+    wchar_t m_lastWChar;
+#endif // SIZEOF_WCHAR_T == 2
+#endif // wxUSE_UNICODE
 
     wxDECLARE_NO_COPY_CLASS(wxTextOutputStream);
 };