[Bug 281516] graphics/libemf: fix build with libc++ 19

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281516

            Bug ID: 281516
           Summary: graphics/libemf: fix build with libc++ 19
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: Individual Port(s)
          Assignee: office@FreeBSD.org
          Reporter: dim@FreeBSD.org
             Flags: maintainer-feedback?(office@FreeBSD.org)
          Assignee: office@FreeBSD.org

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes graphics/libemf to fail to compile with clang 19 and libc++
19, resulting in errors similar to:

  /usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined
template 'std::char_traits<unsigned short>'
    820 |   static_assert(is_same<_CharT, typename
traits_type::char_type>::value,
        |                                          ^
  libemf.cpp:982:30: note: in instantiation of template class
'std::basic_string<unsigned short>' requested here
    982 |     std::basic_string<WCHAR> filename_w( filename, filename +
filename_count );
        |                              ^
  /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
     23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
        |                             ^

libemf should really use char16_t for its 16-bit character type, but the
code is full of Windows assumptions that WCHAR is unsigned short. Since
std::basic_string<unsigned short> is no longer available, use
std::basic_string<char16_t> instead, and cast the result to the
appropriate type for GetEnhMetaFileW().

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

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