Support visual plugins

[caguero]

Supporting visual plugins that act as GUI overlays is a nice way to customize the GUI to the user needs.

[chapulina]

visual plugins that act as GUI overlays is a nice way to customize the GUI to the user needs.

I'll just clarify that Gazebo-classic's GUI overlays are GUI plugins, which are a bit different from visual plugins. We already support custom GUI overlays. Any Ignition GUI plugin can be loaded by Ignition Gazebo, and ignition::gazebo::GuiSystem plugins can access entities and components. Here's an example of the latter.

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Some more pointers for visual plugins that are purely related to rendering:

• ign-gazebo should load <visual><plugin>s. As a reference, here's what it took to load <sensor><plugin>s.
• Gazebo-classic's visual plugins are loaded by gazebo::rendering on both server and client scenes. I believe an equivalent implementation would have to live in ign-rendering. If we want this to be in ign-gazebo, we may need separate plugins for the server side (ignition::gazebo::System) and another one for the client (ignition::gazebo::GuiSystem).
• Modifying visuals on the client should already be possible using an ignition::gazebo::GuiSystem. It's possible to access the rendering engine's singleton and use ign-rendering's API to modify visuals in the rendering thread, using the Render event.
• For accessing the rendering API from a backend system, see Regression for systems that use rendering #36

[chapulina]

The tutorial on #596 explains how to use the rendering API from either server or client.

I believe the only outstanding issue is loading plugins from the <visual><plugin> tag.

[scpeters]

• Gazebo-classic's visual plugins are loaded by gazebo::rendering on both server and client scenes. I believe an equivalent implementation would have to live in ign-rendering.

I've opened gazebosim/gz-rendering#482 to track the ign-rendering side of this.