WebGLPrograms: Set parameters.vertexTangents to true only if necessary.

src/renderers/webgl/WebGLPrograms.js

@@ -163,7 +163,7 @@ function WebGLPrograms( renderer, extensions, capabilities ) {
 
 			combine: material.combine,
 
-			vertexTangents: material.vertexTangents,
+			vertexTangents: ( material.normalMap && material.vertexTangents ),
 			vertexColors: material.vertexColors,
 
 			fog: !! fog,