DMDetector: implement basic support for off-center symbols
DMDetector: implement basic support for off-center symbols

Instead of looking only along the center-line also look for lines above
and below. This can successfully detect samples like the one in #146.

The code is currently disabled though, because it causes a significant
performance drop. Comment out the `tryHarder = false;` line to enable.

huycn/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp