OpenSceneGraph-3.5.1 developer release

OpenSceneGraph-3.5.1, released on 9th November 2015, key deliverables in this dev release are:

• Bug and build fixes
• Added setting of Locator Coordinate System Type in Shapefile plugin
  
• Introduced CMake OSG_PROVIDE_READFILE option for toggling on/off access to the osgDB::rea*File() methods
  
• Replaced all osgDB::read*File() calls with thread safe osgDB::readRef*File() equivalents
  

svn tag: svn co http://svn.openscenegraph.org/osg/OpenSceneGraph/tags/OpenSceneGraph-3.5.1 OpenSceneGraph