Shaders: somehow I need to enable the extension also on ES3.

Although the extension is fully implemented in ES3 itself.

src/Shaders/MeshVisualizer.frag

@@ -63,7 +63,9 @@ in lowp vec3 barycentric;
 out lowp vec4 fragmentColor;
 #endif
 
-#if defined(WIREFRAME_RENDERING) && defined(GL_ES) && __VERSION__ < 300
+/* This is needed also on desktop ES 3.0 emulation on NVidia 330.20 even though
+   fwidth() is part of GLSL ES 3.0 */
+#if defined(WIREFRAME_RENDERING) && defined(GL_ES)
 #extension GL_OES_standard_derivatives : enable
 #endif