Add shadows

[jared-hughes]

The shadow settings need to be tweaked to be enabled.

Also add ShadowMaterial, which allows for some cool effects; see the ground on https://threejs.org/examples/#webgl_geometry_spline_editor.

Probably want to implement planes first to get a similar effect as that example.

I believe this is the final mesh material available easily in three.js without using shader materials (#23).

[jared-hughes]

In https://github.com/jared-hughes/DesThree/commit/c86e927c3d0c4608bf24e2e8d3995b9f6d693b6c, I pretty much used default values, but there were still some shadow artifacts — example graph https://www.desmos.com/calculator/pzp3jcjhhy; try looking at low values of y_0.

After research into shadows, I understand how much of a performance hit they can be. Each light stores a shadow map, created essentially by re-rendering the whole scene from the perspective of the light. Shadows are going to need a lot more thought before being released.

Maybe:

• let users select shadow options (too complex?)
• Default to high-performance, low-quality options
• Default to low-performance, high-quality options
• Disable shadows entirely
• Allow one directional light per scene for shadows
• Hard cap the number of lights with shadows
  • Maybe respond to low FPS by automatically disabling shadows
• Give warnings if too many shadow-enabled lights are present (similar to the "too much detail" warning on vanilla Desmos)

For now, I'm going to drop this issue from the 0.5.2 release.