This adds 2 extra parameters to SpotLight:

- "angle" for controlling light cone width
- "exponent" for controlling sideways light attenuation (from light->target axis towards edges of light cone)

Warning: this changes appearance of everything that used old fake SpotLight. To get approximately similar look use these parameter values:

```
spotLight.angle = Math.PI;
spotLight.exponent = 1;
```

Todo:

- implement spotlights in normal map and skin shaders
- check what's going on with vertex colors and ambient term in unlit parts
- check shadowmap darkening vs unlit parts darkening
- maybe some better formula for light cone attenuation, current one varies too wildly with cone angle