improved matching algorithm

- handle growing matching area together with unchanged screenshots
- search whole area every 6 seconds
$search_ration is 1 when $n is divisible by 6 or 0 (the first iteration) or is close to the end of the iteration.
So we have something like 'full search', 'small search', 'small search', ..., 'full search'. I don't understand why this is better than the linear interpolation.

Is this faster? Behaves better than the current search in the set of needles that is in openqa.opensuse.org? : )