Minor GL optimisation.

modules/juce_opengl/opengl/juce_OpenGLContext.cpp

@@ -158,10 +158,10 @@ public:
         NativeContext::Locker locker (*nativeContext);
 
         JUCE_CHECK_OPENGL_ERROR
-        glViewport (0, 0, viewportArea.getWidth(), viewportArea.getHeight());
 
         if (context.renderer != nullptr)
         {
+            glViewport (0, 0, viewportArea.getWidth(), viewportArea.getHeight());
             context.renderer->renderOpenGL();
             clearGLError();
         }