My motivation is to experiment quirc on macOS, where
neither of SDL1 or V4L2 work.
```
% brew install opencv
% brew install pkg-config
% brew install bsdmake
% bsdmake quirc-demo-opencv
% ./quirc-demo-opencv
```
Note: the macOS version of GNU make (/usr/bin/make) is a bit old
for quirc's Makefile.
Before this patch the `rgb` pointer was incremented by the "inner" for
loop but never reset to the start of the buffer by the "outer" while
loop.
This bug was introduced by the refactoring in
6158aeb30a.
