### 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
XrResult result = xrSuggestInteractionProfileBindings(instance, &suggested_bindings);
	if (result == XR_ERROR_PATH_UNSUPPORTED) {
		
		print_verbose("OpenXR Interaction profile " + ip->name + " is not supported on this runtime");
	} else if (XR_FAILED(result)) {
		print_line("OpenXR: failed to suggest bindings for ", ip->name, "! [", get_error_string(result), "]");
		
	}
	/* For debugging:
	print_verbose("Suggested bindings for " + ip->name);
	for (int i = 0; i < ip->bindings.size(); i++) {
		uint32_t strlen;
		char path[XR_MAX_PATH_LENGTH];
		String action_name = action_get_name(get_action_rid(ip->bindings[i].action));
		XrResult result = xrPathToString(instance, ip->bindings[i].binding, XR_MAX_PATH_LENGTH, &strlen, path);
		if (XR_FAILED(result)) {
			print_line("OpenXR: failed to retrieve bindings for ", action_name, "! [", get_error_string(result), "]");
		}
		print_verbose(" - " + action_name + " => " + String(path));
	}
	*/
	return true;
}
void OpenXRAPI::interaction_profile_free(RID p_interaction_profile) {
	InteractionProfile *ip = interaction_profile_owner.get_or_null(p_interaction_profile);
	ERR_FAIL_NULL(ip);
	ip->bindings.clear();
	interaction_profile_owner.free(p_interaction_profile);
}
bool OpenXRAPI::sync_action_sets(const Vector<RID> p_active_sets) {