### 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
DisplayServer::get_singleton()->set_icon(icon);
						has_icon = true;
					}
				}
			}
			OS::get_singleton()->benchmark_end_measure("game_load");
		}
#ifdef TOOLS_ENABLED
		if (project_manager) {
			OS::get_singleton()->benchmark_begin_measure("project_manager");
			Engine::get_singleton()->set_editor_hint(true);
			ProjectManager *pmanager = memnew(ProjectManager);
			ProgressDialog *progress_dialog = memnew(ProgressDialog);
			pmanager->add_child(progress_dialog);
			sml->get_root()->add_child(pmanager);
			DisplayServer::get_singleton()->set_context(DisplayServer::CONTEXT_PROJECTMAN);
			OS::get_singleton()->benchmark_end_measure("project_manager");
		}
		if (project_manager || editor) {
			/
			Crypto::load_default_certificates(
					EditorSettings::get_singleton()->get_setting("network/tls/editor_tls_certificates").operator String());
		}
#endif
	}
	if (!has_icon && OS::get_singleton()->get_bundle_icon_path().is_empty()) {
		Ref<Image> icon = memnew(Image(app_icon_png));
		DisplayServer::get_singleton()->set_icon(icon);
	}
	if (movie_writer) {