display plugins: make ignition::rendering API calls during ignition::gui::events::Render callback

[scpeters]

After noting some race conditions in the agent info display plugin when clicking the visibility checkbox ( #386 (comment) ), we found the pattern for making ignition::rendering calls from from the screenshot example. The approach is to install an event filter for the MainWindow and then make all ignition::rendering API calls from the eventFilter callback if the event is of type ignition::gui::events::Render. This approach can replace the use of a timer for initializing the rendering objects (see a74261c for example).

The following plugins have rendering API calls outside the eventFilter that could be updated:

• origin_display.cc
• maliput_viewer_plugin.cc

[francocipollone]

Closing as plugins were modified to follow this pattern.