This again leads to a noticeable performance improvement of the
QZXingFilter example, as we get rid of one full copy of the image
data by converting directly into greyscale, instead of first
converting to RGB and then converting to greyscale.
Do not call detaching QByteArray::data, use constData instead.
Also hoist some constants out of the inner conversion loops and make
more variables const.
This allows one to only detect within a certain unscaled region
of the input frame. The reduction of pixels scanned leads to
a significant performance win, and the removal of the scaling
improves the quality of the checked image, thereby leading to
higher detection rates.
