Support decals

[iche033]

The goal is to support adding visual decals on the surface of meshes, e.g. imagine industrial concrete flooring with some spill marks on them. These spill marks can be applied on top of the floor as decals.

2 approaches:

a) Proper decals:
Here's high level description and code snippet for ogre 1.x:
http://wiki.ogre3d.org/OgreDecal

We need to figure out how to do it for ogre 2.1

b) Overlay
Create separate geometry → use texture with alpha transparency → overlay the geometry on top of another.
We need to set rendering order to avoid z-fighting

[ahcorde]

@iche033 can we consider this done ? #188

[iche033]

yep done.

Implemented approach b)