API: Include the region of the match, not just the position

`MatchResult.region` contains all the information that `position` does,
plus indicating the size of the matched area.  This can be useful for OCR
which takes a region to read from.  So with this change it's easy and
convenient to read the text next to a matching icon for instance:

    text = ocr(region=wait_for_match('icon.png').region.extend(right=200))

This changes the arguments `MatchResult` constructor.  This would be a
backwards compatibility break but `MatchResult`s should only be being
created by core stbt code and not by user code.  Still this is something
to watch out for.

Users can still write `pos = match_result.position` so the user facing API
has not been broken.