OpenXR: Add passthrough demo

Author: BastiaanOlij

xr/openxr-passthrough-demo/.gitignore

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+# Godot 4+ specific ignores
+.godot/
+.editorconfig
+/android/
+
+# Ignore vendor plugin add on folder, should be installed by user if needed
+addons/godotopenxrvendors/
+
+# We do want to include editor presets, they are important for configuring standalone headsets
+!export_presets.cfg

xr/openxr-passthrough-demo/README.md

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+# OpenXR Passthrough Demo
+
+This is a demo showing OpenXR's passthrough functionality
+
+Language: GDScript
+
+Renderer: Compatibility
+
+> [!NOTE]
+>
+> This demo requires Godot 4.5 or later
+
+## Screenshots
+
+![Screenshot](screenshots/passthrough_in_quest.png)
+
+## How does it work?
+
+Passthrough is a feature available on a number of XR headsets.
+It uses the cameras on the device to show the real world inside of the headset.
+
+This demo shows how to correctly set up passthrough.
+While the demo will attempt to start in passthrough mode (see the OpenXR Project settings),
+there are extra steps to perform that you will find in `main.gd`.
+The most important being enabling the viewport's transparent background feature.
+
+Note that the environment is also set up to use the sky for ambient color and reflections,
+even after the background has been made transparent.
+While not part of this demo,
+this allows for accurate light estimation in headsets that support this feature.
+
+Finally there is an example of using the `shadow_to_opacity` feature to cut out
+a box shape.
+A use case for this is allowing the user to anchor that box where their keyboard
+is positioned, and in doing so keeping the keyboard visible to the user.
+You can only see the effect properly in headset as the editor viewport does not
+enable a transparent background.
+
+> [!IMPORTANT]
+> In order to save space in this repo, the OpenXR Vendor plugin is not included
+> but related settings are part of this demo!
+> Please install it from the asset library first.
+
+## Action map
+
+This demo project has a barebones action map as we're only dealing with positioning.
+
+## Running on PCVR
+
+This project can be run as normal for PCVR however only a few runtimes actually support passthrough in this way.
+For some you still need to install the OpenXR vendors plugin as passthrough support can rely on vendor extensions.
+
+Ensure that an OpenXR runtime has been installed.
+
+## Running on standalone VR
+
+This demo works best on standalone headsets as many support passthrough.
+
+You **must** install the Android build templates and OpenXR vendors plugin.
+This project does include an `export_preset.cfg` file with example setups for some of these devices.
+Please follow [the instructions for deploying on Android in the manual](https://docs.godotengine.org/en/stable/tutorials/xr/deploying_to_android.html).
+
+

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