### 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
if (!remotefs.is_empty()) {
		int port;
		if (remotefs.contains(":")) {
			port = remotefs.get_slicec(':', 1).to_int();
			remotefs = remotefs.get_slicec(':', 0);
		} else {
			port = 6010;
		}
		Error err = OS::get_singleton()->setup_remote_filesystem(remotefs, port, remotefs_pass, project_path);
		if (err) {
			OS::get_singleton()->printerr("Could not connect to remotefs: %s:%i.\n", remotefs.utf8().get_data(), port);
			goto error;
		}
	}
	if (globals->setup(project_path, main_pack, upwards, editor) == OK) {
#ifdef TOOLS_ENABLED
		found_project = true;
#endif
	} else {
#ifdef TOOLS_ENABLED
		editor = false;
#else
		const String error_msg = "Error: Couldn't load project data at path \"" + project_path + "\". Is the .pck file missing?\nIf you've renamed the executable, the associated .pck file should also be renamed to match the executable's name (without the extension).\n";
		OS::get_singleton()->print("%s", error_msg.utf8().get_data());
		OS::get_singleton()->alert(error_msg);
		goto error;
#endif
	}
	
	{