Mesh: Improve handling of material index.

src/objects/Mesh.js

@@ -264,6 +264,7 @@ Mesh.prototype = Object.assign( Object.create( Object3D.prototype ), {
 								if ( intersection ) {
 
 									intersection.faceIndex = Math.floor( j / 3 ); // triangle number in indexed buffer semantics
+									intersection.face.materialIndex = group.materialIndex;
 									intersects.push( intersection );
 
 								}
@@ -321,6 +322,7 @@ Mesh.prototype = Object.assign( Object.create( Object3D.prototype ), {
 								if ( intersection ) {
 
 									intersection.faceIndex = Math.floor( j / 3 ); // triangle number in non-indexed buffer semantics
+									intersection.face.materialIndex = group.materialIndex;
 									intersects.push( intersection );
 
 								}