Add support for macOS / Metal

[srmainwaring]

Desired behavior

To have ign-rviz working on macOS using the Metal graphics API.

Alternatives considered

Use the OpenGL graphics API available in ogre and ogre2.1. Or use the ROS visualisation version of rviz.

Implementation suggestion

Use the support for macOS / Metal being added to Ignition Garden and provide options or system compile time switches in the ign-rviz plugins to select the graphics API.

There is a work in progress here: https://github.com/srmainwaring/ign-rviz/tree/garden-metal

Additional context

The basic GUI is loading on macOS with Metal. The main issue preventing the app from working is that calls like:

  if (ignition::gui::App()->LoadPlugin("AxesDisplay")) {
    auto axes_plugins =
      ignition::gui::App()->findChildren<ignition::rviz::plugins::MessageDisplayBase *>();

  }

are not finding the loaded plugin correctly for some reason (ie. axes_plugins is empty).