[WIP] Falling leaves working

+ more cleanup
+ fix double-tinted leaves

src/main/kotlin/mods/betterfoliage/util/Geometry.kt

@@ -1,5 +1,6 @@
 package mods.betterfoliage.util
 
+import net.minecraft.client.renderer.Quaternion
 import net.minecraft.util.Direction
 import net.minecraft.util.Direction.*
 import net.minecraft.util.Direction.Axis.*
@@ -66,6 +67,7 @@ data class Double3(var x: Double, var y: Double, var z: Double) {
         val zero: Double3 get() = Double3(0.0, 0.0, 0.0)
         fun weight(v1: Double3, weight1: Double, v2: Double3, weight2: Double) =
             Double3(v1.x * weight1 + v2.x * weight2, v1.y * weight1 + v2.y * weight2, v1.z * weight1 + v2.z * weight2)
+        fun lerp(delta: Double, first: Double3, second: Double3) = first + (second - first) * delta
     }
 
     // immutable operations
@@ -82,6 +84,13 @@ data class Double3(var x: Double, var y: Double, var z: Double) {
         rot.rotatedComponent(SOUTH, x, y, z)
     )
 
+    /** Rotate vector by the given [Quaternion] */
+    fun rotate(quat: Quaternion) =
+        quat.copy()
+            .apply { multiply(Quaternion(x, y, z, 0.0F)) }
+            .apply { multiply(quat.copy().apply(Quaternion::conjugate)) }
+            .let { Double3(it.x, it.y, it.z) }
+
     // mutable operations
     fun setTo(other: Double3): Double3 { x = other.x; y = other.y; z = other.z; return this }
     fun setTo(x: Double, y: Double, z: Double): Double3 { this.x = x; this.y = y; this.z = z; return this }