Work around D3D11 texture pointer getting lost before disposal

[smoogipoo]

The easy-difficulty solution from #53

osu!framework test:

// 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;
using NUnit.Framework;
using osu.Framework.Allocation;
using osu.Framework.Graphics;
using osu.Framework.Graphics.Containers;
using osu.Framework.Graphics.Rendering;
using osu.Framework.Graphics.Sprites;
using osu.Framework.Graphics.Textures;
using osuTK;
using osuTK.Graphics;

namespace osu.Framework.Tests.Visual.Graphics
{
    /// <summary>
    /// Continuously allocates graphics objects and leaves it up to the finalizer to clean them up.
    /// </summary>
    public partial class TestSceneResourceFinalisationStressTest : FrameworkTestScene
    {
        private Drawable? lastConsumer;
        private Func<Drawable>? createConsumer;
        private int counter;

        [Test]
        public void TestTextureConsumer()
        {
            AddStep("enable texture consumer", () => createConsumer = () => new TextureConsumer());
        }

        protected override void Update()
        {
            base.Update();

            if (lastConsumer != null)
            {
                // It's important to not dispose and leave it up to the finalizer.
                Remove(lastConsumer, false);

                if (++counter % 100 == 0)
                {
                    GC.Collect();
                    GC.WaitForPendingFinalizers();
                }
            }

            if (createConsumer != null)
                Add(lastConsumer = createConsumer());
        }

        private partial class TextureConsumer : CompositeDrawable
        {
            private Sprite sprite = null!;

            public TextureConsumer()
            {
                Size = new Vector2(100);
                Anchor = Anchor.Centre;
                Origin = Anchor.Centre;
            }

            [BackgroundDependencyLoader]
            private void load(IRenderer renderer)
            {
                InternalChild = sprite = new Sprite
                {
                    RelativeSizeAxes = Axes.Both,
                    Texture = renderer.CreateTexture(512, 512, true, initialisationColour: Color4.Red),
                };

                sprite.Texture.SetData(new TextureUpload());
            }
        }
    }
}