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/DisjointRectSet.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <AK/Vector.h>
+#include <SharedGraphics/Rect.h>
+
+class DisjointRectSet {
+public:
+    DisjointRectSet() { }
+    ~DisjointRectSet() { }
+    DisjointRectSet(DisjointRectSet&& other) : m_rects(move(other.m_rects)) { }
+
+    void add(const Rect&);
+
+    void clear() { m_rects.clear(); }
+    void clear_with_capacity() { m_rects.clear_with_capacity(); }
+    const Vector<Rect>& rects() const { return m_rects; }
+
+private:
+    void shatter();
+
+    Vector<Rect> m_rects;
+};
+