Add a fence pool for frame tracking fences

[peppy]

Ignoring the fact that these tracking fences are doing nothing in D3D (they are set immediately https://github.com/ppy/veldrid/blob/6a43198af1b069e56ac53770249a5424c1859500/src/Veldrid/D3D11/D3D11GraphicsDevice.cs#L229-L232):

This is a regression so let's fix it locally for the time being.

Time spent in ManualResetEvent construction:

Before this PR:

After this PR:

[smoogipoo]

Is this 16 a magic number? What happens in the (unlikely) even that the pool runs dry, crash on Dequeue()?

[peppy]

Yep. On metal I've never seen usages increase above 1. If we exceed 3 then we're probably going to see texture corruption and stuff elsewhere though? I can use 3 instead of 16 if it feels more robust.

[peppy]

Or we can change it to add to the pool locally, should also be fine.

diff --git a/osu.Framework/Graphics/Veldrid/VeldridRenderer.cs b/osu.Framework/Graphics/Veldrid/VeldridRenderer.cs
index 341a48ea6..7e0b11a9a 100644
--- a/osu.Framework/Graphics/Veldrid/VeldridRenderer.cs
+++ b/osu.Framework/Graphics/Veldrid/VeldridRenderer.cs
@@ -220,9 +220,6 @@ protected override void Initialise(IGraphicsSurface graphicsSurface)
             BufferUpdateCommands = Factory.CreateCommandList();
 
             pipeline.Outputs = Device.SwapchainFramebuffer.OutputDescription;
-
-            for (int i = 0; i < 16; i++)
-                perFrameFencePool.Enqueue(Factory.CreateFence(false));
         }
 
         private Vector2 currentSize;
@@ -296,7 +293,8 @@ protected internal override void FinishFrame()
 
             // This is returned via the end-of-lifetime tracking in `pendingFrameFences`.
             // See `updateLastCompletedFrameIndex`.
-            Fence fence = perFrameFencePool.Dequeue();
+            if (!perFrameFencePool.TryDequeue(out Fence? fence))
+                fence = Factory.CreateFence(false);
 
             Commands.End();
             Device.SubmitCommands(Commands, fence);

[peppy]

I think we still want this in, at least for now. Can potentially be reverted if we decide we don't want end-of-frame fencing on all platforms.