Revert "Remove glewexperimental flag"

This reverts commit 7516dc920a.

src/graphics/glwrap.cpp

@@ -105,6 +105,10 @@ void initGL()
     if (is_gl_init)
         return;
     is_gl_init = true;
+    // For Mesa extension reporting
+#ifdef  __linux__ 
+    glewExperimental = GL_TRUE;
+#endif
     GLenum err = glewInit();
     if (GLEW_OK != err)
         Log::fatal("GLEW", "Glew initialisation failed with error %s", glewGetErrorString(err));
@@ -758,4 +762,4 @@ const std::string getGLExtensions()
     }
 
     return result;
-}   // getGLExtensions
+}   // getGLExtensions