Use lighting and shadows for better perception of scene structure

[blairmacintyre]

The current default flat rendering style, with fixed lights, makes it very difficult to see what's happening in the scene.

It would be great to add two things:

• shadows. If the scene threw shadows it would be much easier to understand.
• extra lights. We could do one or more of the following: light attached to head (headlight), light attach to controller (flashlight) or a movable light the user can reposition in the scene (lamp). Each has an advantage, but with or without shadows, they would help make the scene clearer.

[Utopiah]

Just made the simplest of brush to add extra lights http://vatelier.net/MyDemo/apainter-amaze2017/a-painter-amaze/src/brushes/light.js and even though it "works" I find it difficult to judge. Without #73 or being able to move it after adding it it's a bit tricky.

[Utopiah]

Also for shadows #14 ;)