Fix huge allocation overhead in UnstableRateCounter

[EVAST9919]

Overview

Currently UnstableRateCounter is allocationg a new array of size of judged objects on each judgement, which quickly becoming a problem for maps with many objects. By using Welford's online algorithm we can avoid selection and multiple passes over hitEvents and compute UR in a single loop.

master	pr

Benchmark

Method	Mean	Error	StdDev	Gen0	Allocated
BenchmarkOldUR	62.75 us	0.079 us	0.066 us	8.0566	16968 B
BenchmarkNewUR	29.24 us	0.033 us	0.027 us	0.0305	96 B

Code

// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
// See the LICENCE file in the repository root for full licence text.

using System.Collections.Generic;
using BenchmarkDotNet.Attributes;
using osu.Framework.Utils;
using osu.Game.Rulesets.Objects;
using osu.Game.Rulesets.Scoring;

namespace osu.Game.Benchmarks
{
    public class BenchmarkUR : BenchmarkTest
    {
        private List<HitEvent> events = null!;

        public override void SetUp()
        {
            base.SetUp();
            events = new List<HitEvent>();

            for (int i = 0; i < 1000; i++)
                events.Add(new HitEvent(RNG.NextDouble(-200.0, 200.0), RNG.NextDouble(1.0, 2.0), HitResult.Great, new HitObject(), null, null));
        }

        [Benchmark]
        public void BenchmarkOldUR()
        {
            events.CalculateUnstableRateOld();
        }

        [Benchmark]
        public void BenchmarkNewUR()
        {
            events.CalculateUnstableRate();
        }
    }
}

Algorithm comparison

There is a UnstableRateTest test scene but just in case I compared old and new algo manually, results are basically the same:

[bdach]

Benchmark results are a bit more interesting here but not changed that much.

Method	Mean	Error	StdDev	Gen0	Allocated
CalculateUnstableRate (old)	27.08 us	0.418 us	0.890 us	1.3428	16.54 KB
CalculateUnstableRate (new)	37.06 us	2.236 us	6.593 us	42.14 us	96 B