Updated builds.

build/three.js

@@ -25737,7 +25737,7 @@ THREE.GeometryUtils = {
 
 	// Merge two geometries or geometry and geometry from object (using object's transform)
 
-	merge: function ( geometry1, object2 /* mesh | geometry */ ) {
+	merge: function ( geometry1, object2 /* mesh | geometry */, materialIndexOffset ) {
 
 		var matrix, normalMatrix,
 		vertexOffset = geometry1.vertices.length,
@@ -25750,6 +25750,8 @@ THREE.GeometryUtils = {
 		uvs1 = geometry1.faceVertexUvs[ 0 ],
 		uvs2 = geometry2.faceVertexUvs[ 0 ];
 
+		if ( materialIndexOffset === undefined ) materialIndexOffset = 0;
+
 		if ( object2 instanceof THREE.Mesh ) {
 
 			object2.matrixAutoUpdate && object2.updateMatrix();
@@ -25825,7 +25827,7 @@ THREE.GeometryUtils = {
 
 			}
 
-			faceCopy.materialIndex = face.materialIndex;
+			faceCopy.materialIndex = face.materialIndex + materialIndexOffset;
 
 			faceCopy.centroid.copy( face.centroid );