Fix the toggle of the diagonal kernel activation and code style

When useDiagonal was true, we didn't use the diagonal, the opposite when false.
Also change stepY to use the height of the renderTexture instead of copying the stepX.
This already worked because the texture used was squared so it's just to be careful.
Merge pull request #109 from Teybeo/bug/fix_sharpen_diagonal

Mics fixes for sharpen example