work-around for buggy urdf parser for visual materials

In urdf::Link, only the first visual's material comprises actual color values, all others only have the name.
Thus to correctly show the color of all visuals, we need to search for the first visual with given material name,
much like ros-visualization#1079 has done. If the urdfdom fix (ros/urdfdom#114) is released,
this work-around can be reverted.

Author: rhaschke

src/rviz/robot/robot_link.cpp

@@ -442,6 +442,17 @@ Ogre::MaterialPtr RobotLink::getMaterialForLink( const urdf::LinkConstSharedPtr&
 {
   Ogre::MaterialPtr mat = Ogre::MaterialPtr(new Ogre::Material(nullptr, "robot link material", 0, ROS_PACKAGE_NAME));
 
+  // only the first visual's material actually comprises color values, all others only have the name
+  // hence search for the first visual with given material name (better fix the bug in urdf parser)
+  if (material && !material->name.empty()) {
+    for(const auto& visual : link->visual_array)
+    {
+      if (visual->material_name == material->name) {
+        material = visual->material;
+        break;
+      }
+    }
+  }
   if (!material && link->visual && link->visual->material)
     material = link->visual->material; // fallback to visual's material