Add fixes and improvements to work with C++ code

[nextgeniuspro]

Hi @gyrovorbis, initially I wanted to add only one fix related atomic_bool variable declaration that generates compilation error when you compiles library under C++. However, I found it's kind of uncomfortable to use with necessity to use setup_func in start function. So, here are several fixes I made for this wonderful lib

• Fixed compilation error for atomic_bool under C++
• Removed setup_func from vmu_profiler_start function and instead I return pointer to vmu_profiler_t in this function
• I added user_data to vmu_profiler_measurement so that you can get pointer to needed object inside of get_measure_callbacxk
• I also removed fps_frame and fps_frames[] variables from vmu_profiler_t and code that used them in vmu_profiler_update, since I think this should be made on client side

My changes brake backward compatibility but I believe it's not a big deal. Here is an example how I use this new format in my code

```
vmu_profiler *profiler = vmu_profiler_start(&vmu_config);
if (profiler) {
    auto fps_callback = +[](vmu_profiler_measurement_t *m) {
        Engine* engine = static_cast<Engine*>(m->user_data);
        if (!engine) {
            return;
        }

        m->fstorage = engine->GetRenderContext()->GetFPS();
    };

    vmu_profiler_add_measure(profiler, init_measurement(" FPS", use_float, fps_callback, this));
}