http://forum.unity3d.com/threads/144521-Painting
http://docs.unity3d.com/Documentation/ScriptReference/RaycastHit-textureCoord.html
http://unitycoder.com/blog/2011/08/09/painting-holes-with-shader-v1-0/
http://forum.unity3d.com/threads/35717-Telestrator-style-overlay
http://forum.unity3d.com/threads/22722-UnityPaint-Painting-with-Unity