fix fog, uncommented shader macro

src/mol-gl/shader/chunks/apply-fog.glsl

@@ -1,6 +1,6 @@
-// #ifdef dUseFog
+#ifdef dUseFog
 	float depth = length(vViewPosition);
     // float depth = gl_FragCoord.z / gl_FragCoord.w;
     float fogFactor = smoothstep(uFogNear, uFogFar, depth);
 	gl_FragColor.rgb = mix(gl_FragColor.rgb, uFogColor, fogFactor);
-// #endif
+#endif