Replace GLEW and fix Windows support.

[mrsteyk]

This replaces system-wide GLEW search with vendored glad and ghetto fixes Windows weirdness because C++ sucks.

WIN32_LEAN_AND_MEAN does not include Windows' XML and NOMINMAX prevents min/max defines which break std::max and std::min.
#undef assert is because utfcpp detects C++ edition in a weird way and Windows' define breaks whatever it wants.
ini lib already has an overload for wchar_t null-term. strings, templating is a simple way to get it to work, one could also do .string().c_str() but meh.