Method ptr_from_offset

tutorials/common/imgui/imgui_internal.h:700–700

    T*      end()                       { return (T*)(void*)(Buf.Data + Buf.Size); }
    int     offset_from_ptr(const T* p) { IM_ASSERT(p >= begin() && p < end()); const ptrdiff_t off = (const char*)p - Buf.Data; return (int)off; }
    T*      ptr_from_offset(int off)    { IM_ASSERT(off >= 4 && off < Buf.Size); return (T*)(void*)(Buf.Data + off); }
    void    swap(ImChunkStream<T>& rhs) { rhs.Buf.swap(Buf); }

};