Performance optimization & bugfix

Only depth sort if we need to.
Fixed a bug where if you left the highest ancestor of a slider and re-entered it would still be sliding.

include/JUI/Base/Widget.hpp

@@ -81,6 +81,9 @@ namespace JUI {
         std::vector<Widget*> GetDescendants();
         /// Returns a flat list of all nodes that are higher in the hierarchy list.
         std::vector<Widget*> GetAncestors();
+
+        /// @returns the highest ancestor which is not the scene root.
+        Widget* GetHighestNonRootAncestor();
         /// Returns the nodes directly descendant to this widget.
         std::vector<Widget*> GetChildren();
 
@@ -123,7 +126,7 @@ namespace JUI {
         /// @note The range of valid values is -MAX_INT to MAX_INT.
         /// @note This does not manipulate the OpenGL Z buffer, rather, when rendering,
         /// we sort the direct children of a widget widget and render.
-        int ZIndex() const;
+        [[nodiscard]] int ZIndex() const;
 
         /// Sets this widgets z-index.
         /// @see ZIndex().
@@ -384,6 +387,7 @@ namespace JUI {
         Widget* next = nullptr;
         Widget* prev = nullptr;
         int zindex = 0;
+        bool z_dirty = false;
         int layout_order = 0;
         Vector2 viewport_size{0,0};