Restart the spiral iterator of the Multiply test when maxing out the grid size

[shallawa]

https://github.com/WebKit/MotionMark/issues/52

Refactor the spiral iterator into a separate class. Keep calling its next() method to get the coordinates ofthe next cell. When its isDone() returns true create a new iterator and make the tiles of upper layers have less lightness than the ones on the lower layers.

Add a new class for the roundedRect tile called "Tile". This class can handle the location, size and animation of the Tile.