### 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
register_core_settings(); 
	translation_server = memnew(TranslationServer);
	performance = memnew(Performance);
	GDREGISTER_CLASS(Performance);
	engine->add_singleton(Engine::Singleton("Performance", performance));
	
	
	GLOBAL_DEF_RST("application/run/flush_stdout_on_print", false);
	GLOBAL_DEF_RST("application/run/flush_stdout_on_print.debug", true);
	MAIN_PRINT("Main: Parse CMDLine");
	
	List<String> args;
	List<String> main_args;
	List<String> user_args;
	bool adding_user_args = false;
	List<String> platform_args = OS::get_singleton()->get_cmdline_platform_args();
	
	for (int i = 0; i < argc; i++) {
		args.push_back(String::utf8(argv[i]));
	}
	
	for (const String &arg : platform_args) {
		args.push_back(arg);
	}
	List<String>::Element *I = args.front();
	while (I) {
		I->get() = unescape_cmdline(I->get().strip_edges());
		I = I->next();
	}
	String display_driver = "";