Shaders: fix MeshVisualizer on ES.

The extension line must appear before any other non-preprocessor token.

src/Magnum/Shaders/MeshVisualizer.frag

@@ -36,6 +36,12 @@
 #define layout(arg)
 #endif
 
+/* 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
+
 #ifndef GL_ES
 layout(location = 2) uniform vec4 color = vec4(1.0, 1.0, 1.0, 1.0);
 #else
@@ -64,12 +70,6 @@ in lowp vec3 barycentric;
 out lowp vec4 fragmentColor;
 #endif
 
-/* 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
-
 void main() {
     #ifdef WIREFRAME_RENDERING
     #ifndef NO_GEOMETRY_SHADER