Small allocation fix in fbx loader

[FlorianBorn71]

This PR addresses a problem with loading very large binary fbx files. We had a case where the parsing code performed one billion(!) single small allocation which eventually crashed on any system.
This PR introduces class StackAllocator which can be used to allocate many small objects in a deque pattern. After parsing, the single objects are not de-allocated individually but instead the whole allocator is discarded.
This fix not only allows for loading much larger fbx files (in our case ~16GB with 3 million individual parts) but also improves parsing speed significantly on all other medium to large fbx files.

[turol]

First, StackAllocator should be made noncopyable to avoid mishaps. Second, is this actually safe? It never runs destructors and the allocated objects are not trivial. LazyObject contains a std::unique_ptr and Connection contains a std::string. This sounds like a memory leak to me.

[FlorianBorn71]

Thanks Turo, indeed the current version leaks memory - I will have to re-iterate

[FlorianBorn71]

proper deallocation handled in new commit

[kimkulling]

The stack-allocator looks great. I would recommend not to include a cpp-file. If you need this for linkage just define this in an inline file and include this in the header.

[FlorianBorn71]

Hi @kimkulling, the compilation issues above do not make too much sense, it looks like it's compiling with a cached old .h file.
Is there a way to trigger a clean build?
[EDIT] nm, I think it was a merge issue, it did not take my modified version of the file. Fixing...

[RedSkittleFox]

Object construction and destruction should be delegated to the function related to the allocator. Generally it would be better not to use macross like that overall.

// #include <new>

public:
    template<class T, class... Args>
    T* ConstructObject(Args&&... args)
    {
        return ::new(this->Allocate(sizeof(T))) T(std::forward<Args>(args)...);
    }

    template<class T>
    void DestroyObject(T* obj) noexcept
    {
        obj->~T();
    }

[RedSkittleFox]

This function returns unaligned memory. It should take in additional argument called alignment and align memory internaly.

StackAllocator sa;

char* c = new (sa.Allocate(sizeof(char))) char(1);
std::string* s = new (sa.Allocate(sizeof(std::string))) std::string("str"); // This is UB?
std::cout << reinterpret_cast<size_t>(s) % alignof(std::string) << '\n';

[kimkulling]

Will get fixed by #5096