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.

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.