add support for view3d

[sritchie]

This is going to require letting view3d have children.

;; https://jsxgraph.org/docs/symbols/Functiongraph3D.html

(let [box [-5 5]
      F  (fn [x y] (Math/sin (* x (/ y 4))))]
  [jsx/JSXGraph {:boundingbox [-8 8 8 -8] :showCopyright false :axis false :showNavigation false}
   [:view3d
    {:parents [[-6, -3] [8 8]
               [box box box]]
     :xPlaneRear {:visible false}
     :yPlaneRear {:visible false}}
    [jsx/FunctionGraph3D
     {:parents [F box box]
      :strokeWidth 0.5
      :stepsU 70
      :stepsV 70}]]])