### 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
Vector<const char *> extension_ptrs;
	for (int i = 0; i < enabled_extensions.size(); i++) {
		print_verbose(String("OpenXR: Enabling extension ") + String(enabled_extensions[i]));
		extension_ptrs.push_back(enabled_extensions[i].get_data());
	}
	
	String project_name = GLOBAL_GET("application/config/name");
	
	XrApplicationInfo application_info{
		"", 
		1, 
		"Godot Game Engine", 
		VERSION_MAJOR * 10000 + VERSION_MINOR * 100 + VERSION_PATCH, 
		XR_CURRENT_API_VERSION 
	};
	void *next_pointer = nullptr;
	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
		void *np = wrapper->set_instance_create_info_and_get_next_pointer(next_pointer);
		if (np != nullptr) {
			next_pointer = np;
		}
	}
	XrInstanceCreateInfo instance_create_info = {
		XR_TYPE_INSTANCE_CREATE_INFO, 
		next_pointer, 
		0, 
		application_info, 
		0, 
		nullptr, 
		uint32_t(extension_ptrs.size()),