Fixed repeat for gradients

[DGruenwald]

I realized that I totally botched the repeat for the interpolator when I added the gradient-map filter and fixed it here.

I also tried to add a mirrored repeat mode but, while it usually works like intended, sometimes it flips one pixel too early or late and returns a wrong color:

I think this is a precision issue, since, when the color positions get changed a little, it doesn't happen:

So the question is now, is there a solution to this or should I scrap it?

[DGruenwald]

I decided to remove the mirrored repeat because of the bug above.

[mrdoob]

Sounds good. The simpler the code the better :)