try perturbing vertices to remove slice degeneracies

modules/mesh/slice raster

@@ -422,6 +422,7 @@ function worker() {
       //
       // function to rotate point
       //
+      var delta = (xmax-xmin)/(10*w)
       function rotate(x,y,z) {
          var x1 = x
          var y1 = Math.cos(rx)*y-Math.sin(rx)*z
@@ -432,6 +433,13 @@ function worker() {
          var x3 = Math.cos(rz)*x2-Math.sin(rz)*y2
          var y3 = Math.sin(rz)*x2+Math.cos(rz)*y2
          var z3 = z2
+         //
+         // perturb by lattice resolution
+         //    to remove slice degeneracies
+         //
+         x3 += delta
+         y3 += delta
+         z3 += delta
          return([x3,y3,z3])
          }
       //