[Ogre] Stereo toggler & Handle stereo in the core compositor.

What does this MR do?

Adds an action to enable/disable stereo in an ogre layer.

Refactors the stereo mode to be handled by the core compositor.

Fixes some adaptors that crashed when restarting.

How to test it?

• Launch PoCStereo
• Load an image or a model (File -> load image/model)
• Toggle stereo using View -> Enable stereo

⚠ Fullscreen rendering does not work on macOS, you need to disable it manually in the app config.

Data

Load any vtk image or model.

Dependencies

• !38 (merged)