### 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
Array actions = xr_action_set->get_actions();
			for (int j = 0; j < actions.size(); j++) {
				Ref<OpenXRAction> xr_action = actions[j];
				PackedStringArray toplevel_paths = xr_action->get_toplevel_paths();
				Vector<Tracker *> trackers_for_action;
				for (int k = 0; k < toplevel_paths.size(); k++) {
					
					if (openxr_api->is_top_level_path_supported(toplevel_paths[k])) {
						Tracker *tracker = find_tracker(toplevel_paths[k], true);
						if (tracker) {
							trackers_for_action.push_back(tracker);
						}
					}
				}
				
				if (trackers_for_action.size() > 0) {
					Action *action = create_action(action_set, xr_action->get_name(), xr_action->get_localized_name(), xr_action->get_action_type(), trackers_for_action);
					if (action) {
						
						xr_actions[xr_action] = action;
					}
				}
			}
		}
		
		Array interaction_profile_array = action_map->get_interaction_profiles();
		for (int i = 0; i < interaction_profile_array.size(); i++) {
			Ref<OpenXRInteractionProfile> xr_interaction_profile = interaction_profile_array[i];