Fix map display for moving TF frame (#483)

Instead of the current time, use Time(0) to get the latest available transform as a fallback.
This is the same logic that is applied in RViz from ROS 1.

Resolves #332

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rviz_default_plugins/src/rviz_default_plugins/displays/map/map_display.cpp

@@ -572,10 +572,11 @@ void MapDisplay::transformMap()
 
   Ogre::Vector3 position;
   Ogre::Quaternion orientation;
-  if (!context_->getFrameManager()->transform(frame_, transform_time, current_map_.info.origin,
-    position, orientation) &&
-    !context_->getFrameManager()->transform(frame_, context_->getClock()->now(),
-    current_map_.info.origin, position, orientation))
+  if (!context_->getFrameManager()->transform(
+      frame_, transform_time, current_map_.info.origin, position, orientation) &&
+    !context_->getFrameManager()->transform(
+      frame_, rclcpp::Time(0, 0, context_->getClock()->get_clock_type()),
+      current_map_.info.origin, position, orientation))
   {
     setMissingTransformToFixedFrame(frame_);
     scene_node_->setVisible(false);