### 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
#endif
			} else {
				/
				parsed_pair = false;
			}
			if (parsed_pair) {
				i++;
			}
		}
#ifdef TOOLS_ENABLED
		/
		else if (args[i] == "--doctool") {
			doc_tool_path = ".";
		}
#endif
	}
	uint64_t minimum_time_msec = GLOBAL_DEF(PropertyInfo(Variant::INT, "application/boot_splash/minimum_display_time", PROPERTY_HINT_RANGE, "0,100,1,or_greater,suffix:ms"), 0);
	if (Engine::get_singleton()->is_editor_hint()) {
		minimum_time_msec = 0;
	}
#ifdef TOOLS_ENABLED
#ifdef MODULE_GDSCRIPT_ENABLED
	if (!doc_tool_path.is_empty() && !gdscript_docs_path.is_empty()) {
		DocTools docs;
		Error err;
		Vector<String> paths = get_files_with_extension(gdscript_docs_path, "gd");
		ERR_FAIL_COND_V_MSG(paths.size() == 0, false, "Couldn't find any GDScript files under the given directory: " + gdscript_docs_path);
		for (const String &path : paths) {
			Ref<GDScript> gdscript = ResourceLoader::load(path);
			for (const DocData::ClassDoc &class_doc : gdscript->get_documentation()) {