3 textures blending fix

While working with the terrain blending, I found some artifacts when having more than 2 textures blended together.
Steve_Yorkshire in IRC confirmed that he had also seen the issue, so I dug into it and found that the textures aren't blended properly. This PR fixes specifically the blending between the 2nd and the third texture.

Image of the issue here:
![Blend artifacts](http://imagizer.imageshack.us/a/img822/2708/7zci.png)

With this PR it looks like this:
![Blend fixed](http://imagizer.imageshack.us/a/img823/3235/z5w9.png)