Fixes issue with headless mode

[Delsin-Yu]

Fixes #5

Currently, some APIs inside GDTask rely on the Engine.GetFramesDrawn() API, which always returns 0 when Engine runs with --disable-render-loop, this PR replaces all usage mentioned with Engine.GetProcessFrames(), which works regardless whether the render loop is enabled.
This API is also a better candidate for Unity's Time.frameCount API.

However, Engine.GetProcessFrames() returns ulong, which does not have -1 as a fallback value, I have replaced all -1s with 0ul as a compromise, this could be an issue for some edge cases. Currently, I can pass the builtin test, but I think further tests are required.

// UniTask: PlayerLoopTimer.cs, line 159

protected override bool MoveNextCore()
{
    if (elapsed == 0.0f)
    {
        if (initialFrame == Time.frameCount)
        {
            return true;
        }
    }

    elapsed += Time.deltaTime;
    if (elapsed >= interval)
    {
        return false;
    }

    return true;
}

protected override void ResetCore(TimeSpan? interval)
{
    this.elapsed = 0.0f;
    this.initialFrame = PlayerLoopHelper.IsMainThread ? Time.frameCount : -1;
    if (interval != null)
    {
        this.interval = (float)interval.Value.TotalSeconds;
    }
}

// GDTask: PlayerLoopTimer.cs, line 155

protected override bool MoveNextCore()
{
    if (elapsed == 0.0)
    {
        if (initialFrame == Engine.GetProcessFrames())
        {
            return true;
        }
    }

    elapsed += GDTaskPlayerLoopAutoload.Global.DeltaTime;
    if (elapsed >= interval)
    {
        return false;
    }

    return true;
}

protected override void ResetCore(TimeSpan? interval)
{
    this.elapsed = 0.0;
    this.initialFrame = GDTaskPlayerLoopAutoload.IsMainThread ? Engine.GetProcessFrames() : 0ul;
    if (interval != null)
    {
        this.interval = (float)interval.Value.TotalSeconds;
    }
}

[Delsin-Yu]

Addresses gdtask-no-yield

[Atlinx]

I think the purpose of the -1 was to ensure non-main-thread tasks relied on manual interval-based ticking instead of the engine's process/update frames. Maybe we could set a boolean flag that marks if a loop timer is in the main thread or not. Then we could try something like:

// PlayerLoopTimer.cs

protected override bool MoveNextCore()
{
    if (elapsed == 0.0f)
    {
        if (isMainThread && initialFrame == Engine.GetProcessFrames()) // Check the flag
        {
            return true;
        }
    }
    ...
}

protected override void ResetCore(TimeSpan? interval)
{
    this.elapsed = 0.0;
    this.isMainThread = GDTaskPlayerLoopAutoload.IsMainThread; // Set the flag
    this.initialFrame = Engine.GetProcessFrames();
    if (interval != null)
    {
        this.interval = (float)interval.Value.TotalSeconds;
    }
}

[Atlinx]

Thanks for the PR! ❤️