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.cpp

@@ -0,0 +1,46 @@
+#include <SharedGraphics/DisjointRectSet.h>
+
+void DisjointRectSet::add(const Rect& new_rect)
+{
+    for (auto& rect : m_rects) {
+        if (rect.contains(new_rect))
+            return;
+    }
+
+    m_rects.append(new_rect);
+    if (m_rects.size() > 1)
+        shatter();
+}
+
+void DisjointRectSet::shatter()
+{
+    Vector<Rect> output;
+    bool pass_had_intersections = false;
+    do {
+        pass_had_intersections = false;
+        output.clear_with_capacity();
+        for (size_t i = 0; i < m_rects.size(); ++i) {
+            auto& r1 = m_rects[i];
+            bool r1_had_intersections = false;
+            for (size_t j = 0; j < m_rects.size(); ++j) {
+                if (i == j)
+                    continue;
+                auto& r2 = m_rects[j];
+                if (!r1.intersects(r2))
+                    continue;
+                pass_had_intersections = true;
+                auto pieces = r1.shatter(r2);
+                for (auto& piece : pieces)
+                    output.append(piece);
+                m_rects.remove(i);
+                for (; i < m_rects.size(); ++i)
+                    output.append(m_rects[i]);
+                goto next_pass;
+            }
+            if (!r1_had_intersections)
+                output.append(r1);
+        }
+next_pass:
+        swap(output, m_rects);
+    } while(pass_had_intersections);
+}

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;
+};
+

SharedGraphics/Rect.cpp

@@ -33,3 +33,47 @@ Rect Rect::united(const Rect& other) const
     rect.set_bottom(max(bottom(), other.bottom()));
     return rect;
 }
+
+Vector<Rect> Rect::shatter(const Rect& hammer) const
+{
+    Vector<Rect> pieces;
+    if (!intersects(hammer)) {
+        pieces.append(*this);
+        pieces.append(hammer);
+        return pieces;
+    }
+    Rect top_shard {
+        x(),
+        y(),
+        width(),
+        hammer.y() - y()
+    };
+    Rect bottom_shard {
+        x(),
+        hammer.y() + hammer.height(),
+        width(),
+        (y() + height()) - (hammer.y() + hammer.height())
+    };
+    Rect left_shard {
+        x(),
+        max(hammer.y(), y()),
+        hammer.x() - x(),
+        min((hammer.y() + hammer.height()), (y() + height())) - max(hammer.y(), y())
+    };
+    Rect right_shard {
+        hammer.x() + hammer.width(),
+        max(hammer.y(), y()),
+        (x() + width() - 1) - (hammer.x() + hammer.width() - 1),
+        min((hammer.y() + hammer.height()), (y() + height())) - max(hammer.y(), y())
+    };
+    if (intersects(top_shard))
+        pieces.append(top_shard);
+    if (intersects(bottom_shard))
+        pieces.append(bottom_shard);
+    if (intersects(left_shard))
+        pieces.append(left_shard);
+    if (intersects(right_shard))
+        pieces.append(right_shard);
+
+    return pieces;
+}

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