WindowServer: Avoid overdraw by shattering dirty rects into unique shards.

The algorithm I came up with is O(n^2) but given the small numbers of rects
we're typically working with, it doesn't really matter. May need to revisit
this in the future if we find ourselves with a huge number of rects.

SharedGraphics/Rect.h

@@ -170,6 +170,8 @@ public:
     Point location() const { return m_location; }
     Size size() const { return m_size; }
 
+    Vector<Rect> shatter(const Rect& hammer) const;
+
     operator WSAPI_Rect() const;
 
     bool operator==(const Rect& other) const