### 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
return (instance != XR_NULL_HANDLE);
}
bool OpenXRAPI::is_running() {
	if (instance == XR_NULL_HANDLE) {
		return false;
	}
	if (session == XR_NULL_HANDLE) {
		return false;
	}
	return running;
}
bool OpenXRAPI::openxr_loader_init() {
#ifdef ANDROID_ENABLED
	ERR_FAIL_COND_V_MSG(openxr_loader_library_handle != nullptr, false, "OpenXR Loader library is already loaded.");
	{
		Error error_code = OS::get_singleton()->open_dynamic_library(OPENXR_LOADER_NAME, openxr_loader_library_handle);
		ERR_FAIL_COND_V_MSG(error_code != OK, false, "OpenXR loader not found.");
	}
	{
		Error error_code = OS::get_singleton()->get_dynamic_library_symbol_handle(openxr_loader_library_handle, "xrGetInstanceProcAddr", (void *&)xrGetInstanceProcAddr);
		ERR_FAIL_COND_V_MSG(error_code != OK, false, "Symbol xrGetInstanceProcAddr not found in OpenXR Loader library.");
	}
#endif
	
	OPENXR_API_INIT_XR_FUNC_V(xrCreateInstance);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateApiLayerProperties);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateInstanceExtensionProperties);
	return true;
}
bool OpenXRAPI::resolve_instance_openxr_symbols() {