### 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
if (!is_interaction_profile_supported(p_ip_path)) {
		return false;
	}
	const OpenXRInteractionProfileMetadata::IOPath *io_path = OpenXRInteractionProfileMetadata::get_singleton()->get_io_path(p_ip_path, p_io_path);
	
	ERR_FAIL_NULL_V_MSG(io_path, false, "OpenXR: Unsupported io path " + String(p_ip_path) + String(p_io_path));
	if (io_path->openxr_extension_name == "") {
		
		return true;
	}
	if (!is_extension_enabled(io_path->openxr_extension_name)) {
		
		print_verbose("OpenXR: IO path " + String(p_ip_path) + String(p_io_path) + " requires extension " + io_path->openxr_extension_name);
		return false;
	}
	return true;
}
void OpenXRAPI::copy_string_to_char_buffer(const String p_string, char *p_buffer, int p_buffer_len) {
	CharString char_string = p_string.utf8();
	int len = char_string.length();
	if (len < p_buffer_len - 1) {
		
		memcpy(p_buffer, char_string.get_data(), len);
		p_buffer[len] = '\0';
	} else {
		memcpy(p_buffer, char_string.get_data(), p_buffer_len - 1);
		p_buffer[p_buffer_len - 1] = '\0';
	}
}
bool OpenXRAPI::create_instance() {