Lab task level in ray tracing chapters does not work

[SaschaWillems]

The cpp code has a define for the lab task level and the tutorial tells you to increase this as you go along. The problem with this: No matter what lab task level you set, the code always uses the same shader (no shadows, no reflections, no transparency). For the lab task level to properly work the tutorial would also need to take that into account at shader (compilation) level.