### 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
RID ip = openxr_api->interaction_profile_create(xr_interaction_profile->get_interaction_profile_path());
			if (ip.is_valid()) {
				openxr_api->interaction_profile_clear_bindings(ip);
				Array xr_bindings = xr_interaction_profile->get_bindings();
				for (int j = 0; j < xr_bindings.size(); j++) {
					Ref<OpenXRIPBinding> xr_binding = xr_bindings[j];
					Ref<OpenXRAction> xr_action = xr_binding->get_action();
					Action *action = nullptr;
					if (xr_actions.has(xr_action)) {
						action = xr_actions[xr_action];
					} else {
						print_line("Action ", xr_action->get_name(), " isn't part of an action set!");
						continue;
					}
					PackedStringArray paths = xr_binding->get_paths();
					for (int k = 0; k < paths.size(); k++) {
						openxr_api->interaction_profile_add_binding(ip, action->action_rid, paths[k]);
					}
				}
				
				openxr_api->interaction_profile_suggest_bindings(ip);
				
				if (interaction_profile_array.has(ip)) {
					interaction_profile_array.push_back(ip);
				}
			}
		}
	}
}
OpenXRInterface::ActionSet *OpenXRInterface::create_action_set(const String &p_action_set_name, const String &p_localized_name, const int p_priority) {