This buffer class can avoid copying strings entirely in UTF-8 build, as
it can write directly to the storage provided by the underlying
std::string, and it also needs to keep the contents in UTF-8, instead of
converting it from the current locale encoding, which was at best
useless, and ensure that it is correct, which is necessary with at least
MSVC, as its CRT can return invalid UTF-8 strings even when using this
encoding.
This finally fixes "PrintfError" unit test under MSW.
ae13c2592e