### Godot version

4.1

### System information

Windows 10 - Godot 4.1.1 - Compatibility

### Issue description

When trying to use the Varjo XR3 headset (in OpenXR mode) with Godot 4 (with OpenXR and passthrough activated), the passthrough does not work, i.e. the scene is rendered on a black background instead of the camera stream.

It seems to me that are two problems in the openxr module, more precisely in the openx_api.cpp file (fixing them solves  the issue) : 

1. The XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT  is only set when the number of layers is > 1, which is fine for the Meta Quest headset but does not work for the Varjo since it only requires one layer. 
2. The environment blend mode XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND  is only really set when the XRFbPassthrough extension is present, which is not the case with the Varjo runtime. 

I am not sure how to fix this without causing issues for other headsets.

1. Would it be an issue to always activate the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT ? 
2. Could blend modes be activated even if they are not supported by an extension ? 





### Steps to reproduce

- Create a Godot 4 project with OpenXR activated in Compatibility mode
- Add the code to activate OpenXr and to enable passthrough : https://docs.godotengine.org/en/stable/tutorials/xr/openxr_passthrough.html#passthrough-extension
- Start the Varjo Base software and activate OpenXR
- Run the Godot project

### Minimal reproduction project

N/A
OS::get_singleton()->print("  --gpu-profile                     Show a GPU profile of the tasks that took the most time during frame rendering.\n");
	OS::get_singleton()->print("  --gpu-validation                  Enable graphics API validation layers for debugging.\n");
#if DEBUG_ENABLED
	OS::get_singleton()->print("  --gpu-abort                       Abort on graphics API usage errors (usually validation layer errors). May help see the problem if your system freezes.\n");
#endif
	OS::get_singleton()->print("  --generate-spirv-debug-info       Generate SPIR-V debug information. This allows source-level shader debugging with RenderDoc.\n");
	OS::get_singleton()->print("  --remote-debug <uri>              Remote debug (<protocol>://<host/IP>[:<port>], e.g. tcp://127.0.0.1:6007).\n");
	OS::get_singleton()->print("  --single-threaded-scene           Scene tree runs in single-threaded mode. Sub-thread groups are disabled and run on the main thread.\n");
#if defined(DEBUG_ENABLED)
	OS::get_singleton()->print("  --debug-collisions                Show collision shapes when running the scene.\n");
	OS::get_singleton()->print("  --debug-paths                     Show path lines when running the scene.\n");
	OS::get_singleton()->print("  --debug-navigation                Show navigation polygons when running the scene.\n");
	OS::get_singleton()->print("  --debug-avoidance                 Show navigation avoidance debug visuals when running the scene.\n");
	OS::get_singleton()->print("  --debug-stringnames               Print all StringName allocations to stdout when the engine quits.\n");
#endif
	OS::get_singleton()->print("  --max-fps <fps>                   Set a maximum number of frames per second rendered (can be used to limit power usage). A value of 0 results in unlimited framerate.\n");
	OS::get_singleton()->print("  --frame-delay <ms>                Simulate high CPU load (delay each frame by <ms> milliseconds). Do not use as a FPS limiter; use --max-fps instead.\n");
	OS::get_singleton()->print("  --time-scale <scale>              Force time scale (higher values are faster, 1.0 is normal speed).\n");
	OS::get_singleton()->print("  --disable-vsync                   Forces disabling of vertical synchronization, even if enabled in the project settings. Does not override driver-level V-Sync enforcement.\n");
	OS::get_singleton()->print("  --disable-render-loop             Disable render loop so rendering only occurs when called explicitly from script.\n");
	OS::get_singleton()->print("  --disable-crash-handler           Disable crash handler when supported by the platform code.\n");
	OS::get_singleton()->print("  --fixed-fps <fps>                 Force a fixed number of frames per second. This setting disables real-time synchronization.\n");
	OS::get_singleton()->print("  --delta-smoothing <enable>        Enable or disable frame delta smoothing ['enable', 'disable'].\n");
	OS::get_singleton()->print("  --print-fps                       Print the frames per second to the stdout.\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Standalone tools:\n");
	OS::get_singleton()->print("  -s, --script <script>             Run a script.\n");
	OS::get_singleton()->print("  --main-loop <main_loop_name>      Run a MainLoop specified by its global class name.\n");
	OS::get_singleton()->print("  --check-only                      Only parse for errors and quit (use with --script).\n");
#ifdef TOOLS_ENABLED