### 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
game_path = GLOBAL_GET("application/run/main_scene");
	}
#ifdef TOOLS_ENABLED
	if (!editor && !project_manager && !cmdline_tool && script.is_empty() && game_path.is_empty()) {
		/
		/
		/
		/
		OS::get_singleton()->alert("Couldn't detect whether to run the editor, the project manager or a specific project. Aborting.");
		ERR_FAIL_V_MSG(false, "Couldn't detect whether to run the editor, the project manager or a specific project. Aborting.");
	}
#endif
	MainLoop *main_loop = nullptr;
	if (editor) {
		main_loop = memnew(SceneTree);
	}
	if (main_loop_type.is_empty()) {
		main_loop_type = GLOBAL_GET("application/run/main_loop_type");
	}
	if (!script.is_empty()) {
		Ref<Script> script_res = ResourceLoader::load(script);
		ERR_FAIL_COND_V_MSG(script_res.is_null(), false, "Can't load script: " + script);
		if (check_only) {
			if (!script_res->is_valid()) {
				OS::get_singleton()->set_exit_code(EXIT_FAILURE);
			} else {
				OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
			}
			return false;
		}