Layers
constructor(all: MutableMap<String, TerrainCollision.Layer> = mutableMapOf(
"solid" to Layer.Solid(PhysicsMaterial(friction = 0.4, restitution = 0.2)),
"fluid" to Layer.Fluid(density = 1.0),
), defaultSolid: String = "solid", defaultFluid: String = "fluid", byBlock: MutableMap<Key, String> = HashMap())