Some layers are obstacles

[alin256]

Love this! It would be cool to have the "boulders" integrate more with the layers so that they feel more realistic? Some other ideas are:

• Different layers alter "randomness"
• Some layers are obstacles (maybe they are thinner and don't cross the entire width of the window

Originally posted by @shiffman in #17 (comment)

[alin256]

From my research in the area, the inclusions of layers called "hard stringers" (Linking this thesis, since I did not find a formal definition https://uis.brage.unit.no/uis-xmlui/handle/11250/301097) are a relevant problem when drilling.

Would be cool to implement them.