Fix potential texture corruption on Veldrid

[frenzibyte]

Resolves noticeable texture corruption on osu!:

It's possible to have a draw frame which processes enqueued texture uploads but doesn't have any draw nodes to render to the screen (i.e. no calls to DrawVertices), therefore the texture upload commands continue to the next frame without being submitted and the staging resources incorrectly return back to the pool. This PR resolves the bug by ensuring texture upload commands are submitted before the end of the frame.

Reproducing this bug requires a fresh game state that has absolutely no draw node. Can be repro'd with this simple code in SampleGameGame:

    public partial class SampleGameGame : Game
    {
        protected override void LoadComplete()
        {
            base.LoadComplete();

            Container<Drawable> test;

            Add(test = new Container
            {
                Anchor = Anchor.Centre,
                Origin = Anchor.Centre,
                RelativeSizeAxes = Axes.Both,
                Scale = new Vector2(4f),
                Alpha = 0f,
                Children = new Drawable[]
                {
                    new SpriteText
                    {
                        Anchor = Anchor.Centre,
                        Origin = Anchor.Centre,
                        Text = "AudioMixer",
                        Font = FrameworkFont.Condensed.With(weight: "Bold"),
                        Colour = FrameworkColour.Yellow,
                    },
                    new Box
                    {
                        RelativeSizeAxes = Axes.Both,
                        Colour = FrameworkColour.BlueDark,
                    },
                    new SpriteText
                    {
                        Anchor = Anchor.Centre,
                        Origin = Anchor.Centre,
                        Text = "AudioMixer",
                        Font = FrameworkFont.Condensed.With(weight: "Bold"),
                        Colour = FrameworkColour.Yellow,
                    },
                }
            });

            test.Delay(100).FadeIn(100);
        }
    }

Before:

After: