text: Title: Implement multiple shadows
  Body: - Implement multiple shadows on planes, using the projection algorithm from #191.
  
  ![Screen Shot 2021-12-15 at 3 40 59 PM](https://user-images.githubusercontent.com/9458157/146147924-c904144f-2a27-4b44-a672-c83c834506ac.png)
  
  ![Screen Shot 2021-12-15 at 3 30 14 PM](https://user-images.githubusercontent.com/9458157/146147969-64cdad41-2142-434f-8a4f-110016092cae.png)
  
  - But there are more to do, such as clip points which are inside the plane.
  
  ![Screen Shot 2021-12-15 at 4 17 25 PM](https://user-images.githubusercontent.com/9458157/146149740-51ea21df-ee68-4c6d-a31f-0f7be83db347.png)
  
  