Changelog:
Replaced the buildLevel() function which used sf::RenderTexture for
creating the background with a function which uses cv::Mat for this
task. Not complete yet.
Finally solved the problem which caused the mainwindow to not display
the image correctly:
The cv::Mat representation was correct, but QT didn't interpret it
correctly because it was grayscale and QT used RGB. The cv::Mat just
needed to be converted to grayscale.
Yeah!