Fixed GraphcicsContext::getMaxContextID so it properly returns the current max contextID.

Fixed the osgviewer's compile context code to account for the above fix.

Added compile context support into osgterrain example.

applications/osgviewer/osgviewer.cpp

@@ -157,7 +157,7 @@ int main(int argc, char** argv)
         int numProcessors = OpenThreads::GetNumberOfProcessors();
         int processNum = 0;
 
-        for(unsigned int i=0; i<osg::GraphicsContext::getMaxContextID(); ++i)
+        for(unsigned int i=0; i<= osg::GraphicsContext::getMaxContextID(); ++i)
         {
             osg::GraphicsContext* gc = osg::GraphicsContext::getOrCreateCompileContext(i);
 
@@ -172,6 +172,6 @@ int main(int argc, char** argv)
         }
     }
 
-   viewer.run();
+    viewer.run();
 
 }