### 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
action_map->create_editor_action_sets();
#endif
	} else {
		String default_tres_name = openxr_api->get_default_action_map_resource_name();
		
		if (ResourceLoader::exists(default_tres_name)) {
			action_map = ResourceLoader::load(default_tres_name);
		}
		
		if (action_map.is_null()) {
			action_map.instantiate();
			action_map->create_default_action_sets();
#ifdef TOOLS_ENABLED
			
			action_map->set_path(default_tres_name, true);
			ResourceSaver::save(action_map, default_tres_name);
#endif
		}
	}
	
	if (action_map.is_valid()) {
		HashMap<Ref<OpenXRAction>, Action *> xr_actions;
		Array action_set_array = action_map->get_action_sets();
		for (int i = 0; i < action_set_array.size(); i++) {
			
			Ref<OpenXRActionSet> xr_action_set = action_set_array[i];
			ActionSet *action_set = create_action_set(xr_action_set->get_name(), xr_action_set->get_localized_name(), xr_action_set->get_priority());
			if (!action_set) {
				continue;
			}