BioBlitz: plants that require shade to grow/function

[Gegy]

We want it to be possible for plants to require shade to grow or function in BioBlitz - this models a bit the concept of agroforestry, and promoting interactions between plants. In this case, you would need to place trees around these plants to provide that shade.

The light engine might not be reliable due to working asynchronously, but checking the heightmap should work well enough.