Add reduction parameter

[fand]

Sometimes rays cause glitches because they pass through objects.
It maybe helpful if we could specify the reduction ratio to lengthen rays.

Example

http://glslsandbox.com/e#40808.0

reduction = 1.0

We see big glitch on the center of the screen...

reduction = 0.7

There is still a little glitch on the bottom, but it's acceptable compared to the former one.