Cycles / Tile Rendering:
* Code refactor of tile ordering to simplify the code and avoid some branching. 
* Changed the Center method, so it really follows center -> corners, instead of the BI method, which was confusing sometimes.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56602 954f8c5b-7b00-dc11-b283-0030488c597c