fix for not having transparency in collision scenes on rviz, backport…

…ing the fix to humble (#2929)

Co-authored-by: Alp Akgun <[email protected]>

moveit_ros/visualization/rviz_plugin_render_tools/src/planning_scene_render.cpp

@@ -109,6 +109,7 @@ void PlanningSceneRender::renderPlanningScene(const planning_scene::PlanningScen
       color.r = c.r;
       color.g = c.g;
       color.b = c.b;
+      color.a = c.a;
       alpha = c.a;
     }
     for (std::size_t j = 0; j < object->shapes_.size(); ++j)