fmt trunc fill character into 16bit, even for wchar_t(linux) or char32_t

[XZiar]

In UTF32, a code point is 4 bytes, but fmt only keeps 2 bytes when parsing it into fill:

fmt/include/fmt/base.h

Lines 800 to 825 in cc2ba8f

	template <typename Char> constexpr auto fill_unit() const -> Char {
	using uchar = unsigned char;
	return static_cast<Char>(static_cast<uchar>(fill_data_[0]) |
	(static_cast<uchar>(fill_data_[1]) << 8));
	}
	FMT_CONSTEXPR void set_fill(char c) {
	fill_data_[0] = c;
	set_fill_size(1);
	}
	template <typename Char>
	FMT_CONSTEXPR void set_fill(basic_string_view<Char> s) {
	auto size = s.size();
	set_fill_size(size);
	if (size == 1) {
	unsigned uchar = static_cast<detail::unsigned_char<Char>>(s[0]);
	fill_data_[0] = static_cast<char>(uchar);
	fill_data_[1] = static_cast<char>(uchar >> 8);
	return;
	}
	FMT_ASSERT(size <= max_fill_size, "invalid fill");
	for (size_t i = 0; i < size; ++i)
	fill_data_[i & 3] = static_cast<char>(s[i]);
	}
	};

meanwhile, UTF8 can keep 4 bytes for fill.

repro: https://godbolt.org/z/qfTKe6W6c

[vitaut]

Fixed in 601be1c, thanks for reporting.