Improve performance of drawing counter track sample points

This patch improves the performance of drawing counter track sample points (rectangles and squares) in both low and high details mode. The table below shows the performance improvement:

![countertrack performance - improvement](https://cloud.githubusercontent.com/assets/2546601/5393518/b86ac79c-812a-11e4-8d92-08121448d9c5.png)

Summary of the table above:

* **Low** details **with** selection improved by **7.8%**
* **High** details **with** selection improved by **4.0%**
* **High** details **without** selection improved by **2.5%**

Note that there is still a significant gap between low details and high details performance.