Add math utility class

KeepersCompound.Lightmapper/MathUtils.cs

@@ -0,0 +1,11 @@
+using System.Numerics;
+
+namespace KeepersCompound.Lightmapper;
+
+public static class MathUtils
+{
+    public static float DistanceFromPlane(Plane plane, Vector3 point)
+    {
+        return Math.Abs(Vector3.Dot(plane.Normal, point) + plane.D) / plane.Normal.Length();
+    }
+}