Remove negative z-index.

sky/engine/core/rendering/style/StyleBoxData.h

Index: sky/engine/core/rendering/style/StyleBoxData.h
diff --git a/sky/engine/core/rendering/style/StyleBoxData.h b/sky/engine/core/rendering/style/StyleBoxData.h
index d3f44a8d518487d871e1631ba44e66f57bdcee5e..70923647639fbb8959caa531be3e819604118c28 100644
--- a/sky/engine/core/rendering/style/StyleBoxData.h
+++ b/sky/engine/core/rendering/style/StyleBoxData.h
@@ -54,7 +54,7 @@ public:
 
     const Length& verticalAlign() const { return m_verticalAlign; }
 
-    int zIndex() const { return m_zIndex; }
+    unsigned zIndex() const { return m_zIndex; }
     bool hasAutoZIndex() const { return m_hasAutoZIndex; }
 
     EBoxSizing boxSizing() const { return static_cast<EBoxSizing>(m_boxSizing); }
@@ -77,7 +77,7 @@ private:
 
     Length m_verticalAlign;
 
-    int m_zIndex;
+    unsigned m_zIndex;
     unsigned m_hasAutoZIndex : 1;
     unsigned m_boxSizing : 1; // EBoxSizing
     unsigned m_boxDecorationBreak : 1; // EBoxDecorationBreak