GLTFLoader: Don't set transparent=true with alphaMode=MASK.

examples/js/loaders/GLTFLoader.js

@@ -1781,20 +1781,20 @@ THREE.GLTFLoader = ( function () {
 
 		var alphaMode = materialDef.alphaMode || ALPHA_MODES.OPAQUE;
 
-		if ( alphaMode !== ALPHA_MODES.OPAQUE ) {
+		if ( alphaMode === ALPHA_MODES.BLEND ) {
 
 			materialParams.transparent = true;
 
+		} else {
+
+			materialParams.transparent = false;
+
 			if ( alphaMode === ALPHA_MODES.MASK ) {
 
 				materialParams.alphaTest = materialDef.alphaCutoff !== undefined ? materialDef.alphaCutoff : 0.5;
 
 			}
 
-		} else {
-
-			materialParams.transparent = false;
-
 		}
 
 		if ( materialDef.normalTexture !== undefined ) {