- Make ImWchar32 unsigned.
- Fix Win32 version of ImFileOpen by including windows.h sooner.
- Make ImGuiIO::AddInputCharacterUTF16() more robust by disallowing illegal
surrogate pairs.
- Allow pushing higher plane codepoints through ImGuiIO::AddInputCharacter().
- Minor cleaning up in the high-plane Unicode support.
- Fix Clang -Wunreachable-code warning