Identify start of new apertures more carefully.

The findCenterPositionsOneTrace routine identifies one trace per call. It starts each time by stepping up through rows looking for the start of an "aperture": a wide-enough and bright-enough span of columns, then tracing the full aperture, marking it in the image, then returning. The logic to continue searching after hitting an old or a failed aperture match was not able to find multiple apertures which started at a given row.

Given the tricky book-keeping and logic, I now pass the next unexamined pixel on the first matched row back to the caller, which then passes that back in to findCenterPositionsOneTrace to initialize the next aperture search.