Prototype AspectRatioPanel UI control

Wrote this as a quicky, needed it for something. Don't feel like stabilizing it yet so it's internal.

Author: PJB3005

Robust.Client/UserInterface/Controls/AspectRatioPanel.cs

@@ -0,0 +1,57 @@
+using System.Numerics;
+using Robust.Shared.Maths;
+
+namespace Robust.Client.UserInterface.Controls;
+
+// TODO BEFORE MAKING PUBLIC:
+// Do we need to constrain the aspect ratio in MeasureOverride() too?
+// Doc comments
+
+/// <summary>
+/// A simple UI control that ensures its children are laid out with a fixed aspect ratio.
+/// </summary>
+internal sealed class AspectRatioPanel : Control
+{
+    public AspectRatio AspectRatio
+    {
+        get;
+        set
+        {
+            field = value;
+            InvalidateArrange();
+        }
+    } = AspectRatio.One;
+
+    protected override Vector2 ArrangeOverride(Vector2 finalSize)
+    {
+        var givenRatio = finalSize.X / finalSize.Y;
+
+        if (!MathHelper.CloseTo(givenRatio, AspectRatio.Ratio))
+        {
+            if (givenRatio < AspectRatio.Ratio)
+            {
+                // Too narrow, derive height from width.
+                finalSize = finalSize with { Y = finalSize.X / AspectRatio.Ratio };
+            }
+            else
+            {
+                // Too wide, derive width from height.
+                finalSize = finalSize with { X = finalSize.Y * AspectRatio.Ratio };
+            }
+        }
+
+        return base.ArrangeOverride(finalSize);
+    }
+}
+
+internal struct AspectRatio(float ratio)
+{
+    public static readonly AspectRatio One = new(1);
+
+    public float Ratio = ratio;
+
+    public static AspectRatio FromWidthHeight(float width, float height)
+    {
+        return new AspectRatio(width / height);
+    }
+}