### 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
ERR_FAIL_COND(navigation_server_3d != nullptr);
	ERR_FAIL_COND(navigation_server_2d != nullptr);
	
	navigation_server_3d = NavigationServer3DManager::new_default_server();
	
	if (!navigation_server_3d) {
		WARN_PRINT_ONCE("No NavigationServer3D implementation has been registered! Falling back to a dummy implementation: navigation features will be unavailable.");
		navigation_server_3d = memnew(NavigationServer3DDummy);
	}
	
	ERR_FAIL_NULL_MSG(navigation_server_3d, "Failed to initialize NavigationServer3D.");
	navigation_server_3d->init();
	
	navigation_server_2d = memnew(NavigationServer2D);
	ERR_FAIL_NULL_MSG(navigation_server_2d, "Failed to initialize NavigationServer2D.");
}
void finalize_navigation_server() {
	ERR_FAIL_NULL(navigation_server_3d);
	navigation_server_3d->finish();
	memdelete(navigation_server_3d);
	navigation_server_3d = nullptr;
	ERR_FAIL_NULL(navigation_server_2d);
	memdelete(navigation_server_2d);
	navigation_server_2d = nullptr;
}
void initialize_theme_db() {
	theme_db = memnew(ThemeDB);
}
void finalize_theme_db() {