PaintBrush: Speed up the bucket tool with smarter use of Vector.

Put together a pretty well-performing queue using a Vector and an offset.
By using the new Vector::shift_left(int) instead of Vector::take_first()
we can avoid shifting the vector contents every time and instead only
do it every so often.

Maybe this could be generalized into a separate class, I'm not sure if it's
the best algorithm though, it's just what I came up with right now. :^)

AK/Vector.h

@@ -298,6 +298,19 @@ public:
         m_capacity = new_capacity;
     }
 
+    void shift_left(int count)
+    {
+        ASSERT(count <= m_size);
+        if (count == m_size) {
+            clear();
+            return;
+        }
+        for (int i = 0; i < m_size - count; ++i) {
+            at(i) = move(at(i + count));
+        }
+        m_size -= count;
+    }
+
     void resize(int new_size)
     {
         if (new_size == size())