What shade value to assign below tree canopies?

[Pakillo]

Coming from #8

We need to discount the number of rays that are intercepted by the tree canopy just above, but keep all the other rays (coming from sides).

Look into rayshader::ray_shade function and try to get the number of rays intercepted vertically by the canopy above.

See also explanation of algorithm here https://www.tylermw.com/throwing-shade/