Remove background obscuration checks.

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

Index: sky/engine/core/rendering/style/StyleDifference.h
diff --git a/sky/engine/core/rendering/style/StyleDifference.h b/sky/engine/core/rendering/style/StyleDifference.h
index eb9be49ba24f8e70876ac2e3595187a11a542c8b..9833fde248cb843dfdef225cc02b83abc4a13e41 100644
--- a/sky/engine/core/rendering/style/StyleDifference.h
+++ b/sky/engine/core/rendering/style/StyleDifference.h
@@ -19,17 +19,10 @@ public:
     };
 
     StyleDifference()
-        : m_needsPaintInvalidation(false)
-        , m_layoutType(NoLayout)
+        : m_layoutType(NoLayout)
         , m_propertySpecificDifferences(0)
     { }
 
-    bool needsPaintInvalidation() const { return m_needsPaintInvalidation; }
-    void setNeedsPaintInvalidation()
-    {
-        m_needsPaintInvalidation = true;
-    }
-
     bool needsLayout() const { return m_layoutType != NoLayout; }
     void clearNeedsLayout() { m_layoutType = NoLayout; }
 
@@ -57,8 +50,6 @@ public:
     void setFilterChanged() { m_propertySpecificDifferences |= FilterChanged; }
 
 private:
-    unsigned m_needsPaintInvalidation : 1;
-
     enum LayoutType {
         NoLayout = 0,
         PositionedMovement,