Directional light scaling might cause confusion

Scaling is not necessary for directional lights and the possibility to scale these might cause some confusion.

As expected:

• If you scale a directional light to 0,0,0 it will emit light along the World's Z axis.
• If you scale a directional light to negative size, it will emit light in opposite direction.

Maybe should disable scaling for directional lights. Or maybe the best solution is to just document this behavior.