added workaround to fix Problems on osx when widgets are drawn in the opengl…

added workaround to fix Problems on osx when widgets are drawn in the opengl scene using qpainter. workaround is to set opacity to 0.99 so the qt rasterizer paintengine is used, which seems to be the only paintengine, that properly works with core profiles (as it uses software rendering)

see https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/merge_requests/291