Allow MinimalScene to use either OpenGL or Metal render systems

Add classes to abstract out render hardware interfaces:
- IgnCameraTextureRhi
- RenderThreadRhi
- TextureNodeRhi

Add implementations for OpenGL and Metal in:
- MinimalSceneRhiOpenGL
- MinimalSceneRhiMetal
- Modify the existing MinimalScene class to forward rendering calls to the (virtual) render hardware interface functions.
- Modify plugin CMakeLists.txt to conditionally compile Metal support if the platform is macOS
- Update Application.cc to support either OpenGL or Metal

Refactor MinimalScene to use GraphicAPI
- Use the GraphicsAPI enum to set the choice of graphics interface (OpenGL or Metal).
- Change the MinimalScene plugin element to <graphics_api>...</graphics_api>

Add documentation and confirm to code style
- Document the OpenGL and Metal render interface classes
- Revert default engine to ogre

Set graphics API to Metal for macOS, otherwise use OpenGL
- Update Application.cc to only use Metal for macOS
- Fix issues with code check

Remove unused variable from Application.cc

Change pointer casts to satisfy code check
- Code check requires we use reinterpret_cast rather than static_cast.

Combine initialisation methods
- Revert to original spelling and names for Initialize()
- In RenderItem call Ready() using invokeMethod when not using Metal
- Attempting to resolve residual QSG threading issue (use of threaded QSG is inconsistent between platforms)

Force both OpenGL and Metal to use QSG_RENDER_LOOP "basic"
- Force QSG to run on the main application thread
- This will resolve the exception on Ubuntu but a better solution is required.

Resolve issue with QSG render loop for Metal
- Metal version now supports "threaded"  QSG_RENDER_LOOP
- OpenGL version still unresolved.

Update src/plugins/minimal_scene/MinimalScene.hh

Co-authored-by: Alejandro Hernández Cordero <ahcorde@gmail.com>

Update src/plugins/minimal_scene/MinimalSceneRhi.hh

Co-authored-by: Alejandro Hernández Cordero <ahcorde@gmail.com>

Fix issues with OpenGL context thread affinity
- The OpenGL context must be moved from the QSGRenderThread to the main thread for initialisation
- After initialisation the OpenGL context is then moved to the Ignition render thread

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

- Suppress cpplint warnings in MinimalSceneRhi and MinimalSceneRhiOpenGL that look like false positives

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

- Update MinimalScene.hh to fix CI error

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

- Update include guard
- Remove comment after function parameter
- Document graphics_api XML element
- Add error message for unsupported graphics APIs

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

- Add list of available graphics APIs
- Set default graphics API to 'metal' if the platform is Apple, otherwise use 'opengl'.

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

Co-authored-by: Alejandro Hernández Cordero <ahcorde@gmail.com>
Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

Co-authored-by: Alejandro Hernández Cordero <ahcorde@gmail.com>
Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>