Remove negative z-index.

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

Index: sky/engine/core/rendering/style/RenderStyle.h
diff --git a/sky/engine/core/rendering/style/RenderStyle.h b/sky/engine/core/rendering/style/RenderStyle.h
index 6332853b7be78553b0839d04bf6671834daf2b88..2b28097c524186c5ae3876c68e5da6ed572b65ea 100644
--- a/sky/engine/core/rendering/style/RenderStyle.h
+++ b/sky/engine/core/rendering/style/RenderStyle.h
@@ -1006,8 +1006,8 @@ public:
 
     bool hasAutoZIndex() const { return m_box->hasAutoZIndex(); }
     void setHasAutoZIndex() { SET_VAR(m_box, m_hasAutoZIndex, true); SET_VAR(m_box, m_zIndex, 0); }
-    int zIndex() const { return m_box->zIndex(); }
-    void setZIndex(int v) { SET_VAR(m_box, m_hasAutoZIndex, false); SET_VAR(m_box, m_zIndex, v); }
+    unsigned zIndex() const { return m_box->zIndex(); }
+    void setZIndex(unsigned v) { SET_VAR(m_box, m_hasAutoZIndex, false); SET_VAR(m_box, m_zIndex, v); }
 
     void setHasAutoWidows() { SET_VAR(rareInheritedData, m_hasAutoWidows, true); SET_VAR(rareInheritedData, widows, initialWidows()); }
     void setWidows(short w) { SET_VAR(rareInheritedData, m_hasAutoWidows, false); SET_VAR(rareInheritedData, widows, w); }