### 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
InteractionProfile *ip = interaction_profile_owner.get_or_null(p_interaction_profile);
	ERR_FAIL_NULL_V(ip, false);
	if (!interaction_profile_supports_io_path(ip->name, p_path)) {
		return false;
	}
	XrActionSuggestedBinding binding;
	Action *action = action_owner.get_or_null(p_action);
	ERR_FAIL_COND_V(action == nullptr || action->handle == XR_NULL_HANDLE, false);
	binding.action = action->handle;
	XrResult result = xrStringToPath(instance, p_path.utf8().get_data(), &binding.binding);
	if (XR_FAILED(result)) {
		print_line("OpenXR: failed to get path for ", p_path, "! [", get_error_string(result), "]");
		return false;
	}
	ip->bindings.push_back(binding);
	return true;
}
bool OpenXRAPI::interaction_profile_suggest_bindings(RID p_interaction_profile) {
	ERR_FAIL_COND_V(instance == XR_NULL_HANDLE, false);
	InteractionProfile *ip = interaction_profile_owner.get_or_null(p_interaction_profile);
	ERR_FAIL_NULL_V(ip, false);
	const XrInteractionProfileSuggestedBinding suggested_bindings = {
		XR_TYPE_INTERACTION_PROFILE_SUGGESTED_BINDING, 
		nullptr, 
		ip->path, 
		uint32_t(ip->bindings.size()), 
		ip->bindings.ptr() 
	};