### 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 (!valid_combination) {
			OS::get_singleton()->print("Invalid renderer/driver combination '%s' and '%s', aborting. %s only supports the following drivers ", rendering_method.utf8().get_data(), rendering_driver.utf8().get_data(), rendering_method.utf8().get_data());
			for (int d = 0; d < available_drivers.size(); d++) {
				OS::get_singleton()->print("'%s', ", available_drivers[d].utf8().get_data());
			}
			OS::get_singleton()->print(".\n");
			goto error;
		}
	}
	default_renderer = renderer_hints.get_slice(",", 0);
	GLOBAL_DEF_RST_BASIC(PropertyInfo(Variant::STRING, "rendering/renderer/rendering_method", PROPERTY_HINT_ENUM, renderer_hints), default_renderer);
	GLOBAL_DEF_RST_BASIC("rendering/renderer/rendering_method.mobile", default_renderer_mobile);
	GLOBAL_DEF_RST_BASIC("rendering/renderer/rendering_method.web", "gl_compatibility"); 
	
	if (rendering_method.is_empty()) {
		rendering_method = GLOBAL_GET("rendering/renderer/rendering_method");
	}
	if (rendering_driver.is_empty()) {
		if (rendering_method == "gl_compatibility") {
			rendering_driver = GLOBAL_GET("rendering/gl_compatibility/driver");
		} else {
			rendering_driver = GLOBAL_GET("rendering/rendering_device/driver");
		}
	}
	
	
	
	OS::get_singleton()->set_current_rendering_driver_name(rendering_driver);
	OS::get_singleton()->set_current_rendering_method(rendering_method);