WindowServer: Only blit dirty rect of windows to back buffer.

Previously we'd blit every pixel in every window that intersected any dirty
rect to the back buffer. With this patch, we limit ourselves to blitting the
pixels inside the actual dirty rects.

There's still a lot of optimizations to make in this code.

SharedGraphics/Rect.h

@@ -18,6 +18,12 @@ public:
         , m_size(size)
     {
     }
+    Rect(const Rect& other)
+        : m_location(other.m_location)
+        , m_size(other.m_size)
+    {
+    }
+
     Rect(const GUI_Rect&);
 
     bool is_null() const