raycastVoxels() default normal to top face if hits inside voxel

voxel-raycast can return a hit normal of [0,0,0] if, for example, the
starting position is within a voxel, but this is not a valid normal
vector. If this happens, instead default to [0,1,0], the top face.

index.js

@@ -280,6 +280,12 @@ Game.prototype.raycastVoxels = function(start, direction, maxDistance, epilson)
   var adjacentPosition = [0, 0, 0]
   var voxelPosition = this.voxelPosition(hitPosition)
   vector.add(adjacentPosition, voxelPosition, hitNormal)
+
+  if (hitNormal[0] + hitNormal[1] + hitNormal[2] !== 1) {
+    hitNormal[0] = 0
+    hitNormal[1] = 1
+    hitNormal[2] = 0
+  }
 
   return {
     position: hitPosition,