Topic shaderhooks - introduced implementable hooks in shader code for more flexible integration of osgearth into rendering systems

While working with osgearth in our rendering system we needed a possibility to influence the osgearth rendering.

This is why we introduced hook functions into the shader code that can be used to modify the overall rendering result. Further we externalized the GLSL version to CMAKE variables 

ADditionally there are some build fixes to assure osgearth to compile on mac and together with osg 3.3 which slightly changed the inheritance of osgGA::GUIEventAdapter


The original Code modification where performed by Jan Ole Vollmer