Fixed blending for 3 terrain textures.
This commit gives a better blending when there is >2 textures blending.
Added the new terrain blending.

Added an implementation of the terrain blending described in this article: http://www.gamasutra.com/blogs/AndreyMishkinis/20130716/196339/Advanced_Terrain_Texture_Splatting.php by Andrew Mishkinis.

Edit: Now textures without a heightmap will blend linearly.
GUI additions for the new terrain blending

This commit adds new GUI elements for activating/modifying the new terrain
blending.