Image::flipVertically(), Image::flipHorizontally() optimizations.

Hi.
I have made a few optimizations regarding these methods. Now they definitely perform better, but I kind of agree these're the micro-optimizations thus it is up to you whether or not to accept that.

Anyway, I benchmarked these methods using big PNG image (2048x2048px):

    #include <SFML/Graphics.hpp>
    #include <ctime>
    #include <iostream>

    int main() {

        sf::Image img;
        int num = 1000, i;
     
        if (img.loadFromFile("test_big_img.png")) {
	    sf::Clock clock;
	    for (i = 0; i < num; ++i) {
	        //img.flipHorizontally();
	        img.flipVertically();
	    }
	    sf::Time elapsed = clock.getElapsedTime();
	    std::cout << "flipVertically: iterations: " << i << " total time: " << elapsed.asSeconds() << " average: " << elapsed.asSeconds() / num << std::endl;
        }

        return 0;
    }

and here're the results:
//g++ -O2 -o bench ./bench.cpp -L/home/varnie/thrash/SFML-2.1/lib -lsfml-graphics -lsfml-window -lsfml-system

//current version: flipHorizontally: iterations: 1000 total time: 32.5636 average: 0.0325636
//new version     : flipHorizontally: iterations: 1000 total time: 13.855 average: 0.013855

//current version: flipVertically: iterations: 1000 total time: 31.548 average: 0.031548
//new version     : flipVertically: iterations: 1000 total time: 8.9526 average: 0.0089526

I have GCC version 4.7.2 

The image I used is located here: https://www.dropbox.com/s/f6co1urplxtgatm/test_big_img.png

Feel free to code-review my changes. Thank you.