Ray lookAt function

@mkkellogg Do you mind double checking that I didn't do anything stupid?

I've removed adjustShadowValue1K. Instead of multiplying every sample by 1000 and adding the bias I'm now dividing dp and subtracting the bias. I'm also using texture2dCompare which uses step() internaly instead of the if condition. The result seems to be identical but, at least on my gpu (Intel HD Graphics 6000), it runs much faster.

fix VRControls onError

Update Lut.js

If in the example `webgl_geometry_colors_lookuptable` I change 
https://github.com/mrdoob/three.js/blob/dev/examples/webgl_geometry_colors_lookuptable.html#L183 to `lut.setMin( -1000 );`, the scale goes from 0 to 3000, not from -1000 to 2000. Anything else than 0 as minimum scale value is broken. This pull request negative

refactoring "derivatives" material property in examples

#7952 New Stencil Buffer Management

Fixed broken link.

Fixed broken link to particles example.