Wide char to UTF-8 conversion drops surrogate pairs

[ptc-tgamper]

The convert function,

fmt/include/fmt/format.h

Line 1409 in 6e462b8

static auto convert(Buffer& buf, basic_string_view<WChar> s,

decodes the surrogate pair,

fmt/include/fmt/format.h

Line 1422 in 6e462b8

c = (c << 10) + static_cast<uint32_t>(*p) - 0x35fdc00;

but does not write the resulting unicode character to the output buffer.

Walked into this when printing a std::filesystem::path on Windows, that uses smiley characters, such as 😀. The character is simply dropped.

[phprus]

Fix: #4095