Moved getenv usage across to safer osg::getEnvVar() usage

src/osg/GraphicsContext.cpp

@@ -18,6 +18,7 @@
 #include <osg/View>
 #include <osg/GLObjects>
 #include <osg/ContextData>
+#include <osg/EnvVar>
 
 #include <osg/FrameBufferObject>
 #include <osg/Program>
@@ -153,10 +154,10 @@ std::string GraphicsContext::ScreenIdentifier::displayName() const
 
 void GraphicsContext::ScreenIdentifier::readDISPLAY()
 {
-    const char* ptr = 0;
-    if ((ptr=getenv("DISPLAY")) != 0)
+    std::string str;
+    if (getEnvVar("DISPLAY", str))
     {
-        setScreenIdentifier(ptr);
+        setScreenIdentifier(str);
     }
 }