### 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
}
				OS::get_singleton()->alert("Error: Invalid MainLoop script base type: " + script_base);
				ERR_FAIL_V_MSG(false, vformat("The global class %s does not inherit from SceneTree or MainLoop.", main_loop_type));
			}
			script_loop->set_script(script_res);
			main_loop = script_loop;
		}
	}
	if (!main_loop && main_loop_type.is_empty()) {
		main_loop_type = "SceneTree";
	}
	if (!main_loop) {
		if (!ClassDB::class_exists(main_loop_type)) {
			OS::get_singleton()->alert("Error: MainLoop type doesn't exist: " + main_loop_type);
			return false;
		} else {
			Object *ml = ClassDB::instantiate(main_loop_type);
			ERR_FAIL_NULL_V_MSG(ml, false, "Can't instance MainLoop type.");
			main_loop = Object::cast_to<MainLoop>(ml);
			if (!main_loop) {
				memdelete(ml);
				ERR_FAIL_V_MSG(false, "Invalid MainLoop type.");
			}
		}
	}
	OS::get_singleton()->set_main_loop(main_loop);
	SceneTree *sml = Object::cast_to<SceneTree>(main_loop);
	if (sml) {
#ifdef DEBUG_ENABLED
		if (debug_collisions) {