### 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
HashMap<StringName, ProjectSettings::AutoloadInfo> autoloads = ProjectSettings::get_singleton()->get_autoload_list();
				/
				for (const KeyValue<StringName, ProjectSettings::AutoloadInfo> &E : autoloads) {
					const ProjectSettings::AutoloadInfo &info = E.value;
					if (info.is_singleton) {
						for (int i = 0; i < ScriptServer::get_language_count(); i++) {
							ScriptServer::get_language(i)->add_global_constant(info.name, Variant());
						}
					}
				}
				/
				List<Node *> to_add;
				for (const KeyValue<StringName, ProjectSettings::AutoloadInfo> &E : autoloads) {
					const ProjectSettings::AutoloadInfo &info = E.value;
					Node *n = nullptr;
					if (ResourceLoader::get_resource_type(info.path) == "PackedScene") {
						/
						Ref<PackedScene> scn;
						scn.instantiate();
						scn->set_path(info.path);
						scn->reload_from_file();
						ERR_CONTINUE_MSG(!scn.is_valid(), vformat("Can't autoload: %s.", info.path));
						if (scn.is_valid()) {
							n = scn->instantiate();
						}
					} else {
						Ref<Resource> res = ResourceLoader::load(info.path);
						ERR_CONTINUE_MSG(res.is_null(), vformat("Can't autoload: %s.", info.path));
						Ref<Script> script_res = res;
						if (script_res.is_valid()) {