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.
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