improved matching algorithm

- handle growing matching area together with unchanged screenshots
- search whole area every 6 seconds
The original behavior is inefficient:
- if the matching area was far from the original location, it spent significant part of the timeout by checking only the small area which did not mach
- if the matching area appears after longer time (say at 75% of timeout), it does near full search all the time - this causes rather high load
- if it goes the standstill code path, the full search is never done

With this patch the matching area is found anywhere, with predictable time and load.

I did not check openqa.opensuse.org, but for autoyast test on sleposbuilder3.suse.cz it is more reliable and faster.