Added support for per context extension string.

Note, this required adding a unsigned int context ID to the osg::isGLUExtensionSupported(,)
and osg::isGLExtensionSupported(,) functions.  This may require reimplementation
of end user code to accomodate the new calling convention.

include/osg/FragmentProgram

@@ -231,13 +231,13 @@ class OSG_EXPORT FragmentProgram : public StateAttribute
         class OSG_EXPORT Extensions : public osg::Referenced
         {
             public:
-                Extensions();
+                Extensions(unsigned int contextID);
 
                 Extensions(const Extensions& rhs);
                 
                 void lowestCommonDenominator(const Extensions& rhs);
                 
-                void setupGLExtenions();
+                void setupGLExtenions(unsigned int contextID);
 
                 void setFragmentProgramSupported(bool flag) { _isFragmentProgramSupported=flag; }
                 bool isFragmentProgramSupported() const { return _isFragmentProgramSupported; }