feat(ui): add RenderOrder property for UI component stacking

sources/engine/Stride.UI/Engine/UIComponent.cs

@@ -129,6 +129,15 @@ public UIPage Page
         [DefaultValue(RenderGroup.Group0)]
         public RenderGroup RenderGroup { get; set; }
 
+        /// <summary>
+        /// Gets or sets the render order of this UI component. Components with a lower value
+        /// are rendered first (behind), higher values are rendered on top.
+        /// </summary>
+        [DataMember(90)]
+        [Display("Render Order")]
+        [DefaultValue(0)]
+        public int RenderOrder { get; set; }
+
         /// <summary>
         /// A fixed size UI component with height of 1 will be this much of the vertical resolution on screen
         /// </summary>

sources/engine/Stride.UI/Rendering/UI/RenderUIElement.cs

@@ -37,6 +37,7 @@ public RenderUIElement()
         public bool IsBillboard;
         public bool SnapText;
         public bool IsFixedSize;
+        public int RenderOrder;
 
         /// <summary>
         /// Last registered position of teh mouse

sources/engine/Stride.UI/Rendering/UI/UIRenderFeature.cs

@@ -100,6 +100,9 @@ private void DrawInternal(RenderDrawContext context, RenderView renderView, Rend
                 uiElementStates.Add(new UIElementState(renderElement));
             }
 
+            // sort by RenderOrder so components with lower value are drawn first (behind)
+            uiElementStates.Sort(static (a, b) => a.RenderObject.RenderOrder.CompareTo(b.RenderObject.RenderOrder));
+
             // evaluate the current draw time (game instance is null for thumbnails)
             var drawTime = game != null ? game.DrawTime : new GameTime();
 

sources/engine/Stride.UI/Rendering/UI/UIRenderProcessor.cs

@@ -52,6 +52,7 @@ public override void Draw(RenderContext gameTime)
                 renderUIElement.IsBillboard = uiComponent.IsBillboard;
                 renderUIElement.SnapText = uiComponent.SnapText;
                 renderUIElement.IsFixedSize = uiComponent.IsFixedSize;
+                renderUIElement.RenderOrder = uiComponent.RenderOrder;
 
                 if (renderUIElement.RenderGroup != uiComponent.RenderGroup)
                 {