Removed setter for furthestPointLayout

The setter would only allow the dimensions of the point to increase. This makes sense in theory, but the method `invalidateLayout` is setting the point to zero, but this will be ignored by the setter as it is not larger than the current value.

An alternate solution to this problem would be to replace 'self.furthestBlockPoint = CGPointZero' with '_furthestBlockPoint = CGPointZero' so that the setter is not used.