### 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
ERR_FAIL_COND_V(!_start_success, false);
	bool has_icon = false;
	String positional_arg;
	String game_path;
	String script;
	String main_loop_type;
	bool check_only = false;
#ifdef TOOLS_ENABLED
	String doc_tool_path;
	bool doc_base = true;
	String _export_preset;
	bool export_debug = false;
	bool export_pack_only = false;
#ifdef MODULE_GDSCRIPT_ENABLED
	String gdscript_docs_path;
#endif
#ifndef DISABLE_DEPRECATED
	bool converting_project = false;
	bool validating_converting_project = false;
#endif 
#endif 
	main_timer_sync.init(OS::get_singleton()->get_ticks_usec());
	List<String> args = OS::get_singleton()->get_cmdline_args();
	for (int i = 0; i < args.size(); i++) {
		/
		/
		/
		if (args[i] == "--check-only") {
			check_only = true;