Add XR demo for spectator view

[BastiaanOlij]

This is a demo project that shows how to implement a spectator view when deploying a VR project to desktop, while ignoring the spectator view code from an export to a standalone headset.

The demo also has additional logic to show how to deal with a 3rd tracker to track a physical camera. While only the basics are shown, this could be used to create a mixed reality setup where we combine a 3rd person render result, with a physical camera recording the player in front of a green screen.

This demo requires the godotengine/godot#110002 (Compatibility) or godotengine/godot#109955 (Vulkan) fix to display left or right eye views on the desktop!

[BastiaanOlij]

Note, there is currently a bug around displaying the subviewport textures if we want to show the left or right eye results: godotengine/godot#100004

However that doesn't stop the main spectator camera from working, optionally showing the left/right eye output is a nice bonus here for this example.

[BastiaanOlij]

Couple of things I should really add here before this is finished:

• add in the ability to zoom in/out
• add a 4th view option for a stabilized first person view (same as I use on stream)
• if current 3rd person spectator camera is used, add ability to see that camera in headset
• if current 3rd person spectator camera is used, add ability to adjust its position by pointing at it with your controller.