WebGLProgram: Moved #extension directive before precision. See #6493.

src/renderers/webgl/WebGLProgram.js

@@ -275,11 +275,11 @@ THREE.WebGLProgram = ( function () {
 
 			prefix_fragment = [
 
+				( parameters.bumpMap || parameters.normalMap || parameters.flatShading ) ? '#extension GL_OES_standard_derivatives : enable' : '',
+
 				'precision ' + parameters.precision + ' float;',
 				'precision ' + parameters.precision + ' int;',
 
-				( parameters.bumpMap || parameters.normalMap || parameters.flatShading ) ? '#extension GL_OES_standard_derivatives : enable' : '',
-
 				customDefines,
 
 				'#define MAX_DIR_LIGHTS ' + parameters.maxDirLights,