风格化NPR渲染管线

[axmand]

maptalks's version and what browser you use?

Issue description

咱们PBR材质已经能较好的体现真实感效果，是否考虑增加NPR风格化渲染管线提供更多的可视化效果呢？

Please provide a reproduction URL (on any jsfiddle like site)